I think part of this may be the relative experience with 'abuse' of this type of spell of those speaking. I've honestly not been in a game with many ring spells. Most people seem to want to have things they can cast 'now' if they have to... rather than things they did yesterday or last month that are still working. ( I'm a last month kind of guy, but ... strangely GMs always seem to pick something that last month's project doesn't work just right on. )
I can't see any particular problems with the main rule. It seems like a good idea.
I guess, there could be some odd contexts in which it might not be obvious what happens.
Say, for example, your characters travel to a regio that is on the surface of a cloud? Can they cast ring duration effects on the cloud surface? Or what if your characters are on an island that is floating in the sea, or on a lake? (I know such a thing is not realistic, but it could well exist in Mythic Europe).
What happens if a ring is cast on fixed earth, but then the earth is teleported to another location? Does the ring effect remain active? Where does it remain active, in the original location, or the where the earth now is?
Of course, such situations are probably rare enough that you can deal with them on a case by case basis.
I see more problems with the exception. The basic problem being what is, and is not, "the deck of a ship".
Some questions:
Is the hold of a ship a "deck"?
Is a crow's nest a "deck"?
Is a raft a "ship"?
Is a dingy a "ship"?
Is the presence of water important --- can mobile rings be cast on "a ship" that is travelling over land or in the air? Obviously, this would be a magical ship.
If mobile rings can be cast on a magical ship travelling over land, can they be cast on the deck of a mundane cart drawn by horses?
If mobile rings can be cast on the deck of a mundane cart drawn by horses, can they be cast on a dinner plate carried by a servant?
In all my years of playing Ars, I've only once - and quite recently even - ever had someone use either Ring or Circle spells. I feel kind of blessed, to have been spared the (potential) abuse of these.
Few wards have ever been used, and then almost solely Ward against Heat and Flames, Range: Personal
It's a philosophical question isn't it? Do you slip a ring onto your finger (so the ring moves) or does a finger-shaped bit of the world move into the immobile ring?
It's not very far from there to think about immobile movers and (Post-)Newton physics concerning gravity. Oh and there is a bit of Einsteinian relativity theory to consider as well...
But then again, the middle ages were a much simpler time (that's why we love them, isn't? Can't be the cold, smell and barbarous mindset!)
We have a ring as having to be physically demarqued so that the ring would have to be etched in chalk or cut into the turf. However the major thing is that ring wards can't be acted upon by any medium powered by the subject of the ward for example a ward vs demons is pointless if they just throw big rocks at you or if you use a propelled knife to scrape away a ward vs humans then it's pretty pointless.
You could manufacture some huge complex scenario where you caused an overhanging tree branch to fall under gravity into the ward (maybe breaking it) but it's thinking around the ward which is rewarded not just trying to excuse ignoring it.
It's a cheap way of enchanting an effect within a wooden or metal hoop (for example, play ring toss with a demon to trap him; "portable hole" spell; ), although it raises all sorts of questions as to what's "in" the ring. If the portable ring is strong enough, basically it's a slightly bulky invested device.
I'm still trying to figure out the horizontal range of effect of a stationary ring drawn perpendicular to the ground, i.e., on a wall. If I drew a PeTe ring destroying the solidity of stone (but no other property) on the wall of a castle, would it effect only the wall the ring is drawn on, or all stone walls forward/behind the ring? Both are reasonable, and while I prefer the former, there's a strong argument for the latter given that there is no vertical bound for rings.
Mobility: Thats the question. When a Magus can rip up the ground (with the circle on it) and transport it...Does that mean it would work?
We have generally allowed that a Hoop (say) can be thrown down, then a spell cast upon it...{Of course, you could enchant the hoop to shrink and grow upon command, but that would not be the 'Ring' spell.}
Once the item is MOVED, the 'ring' spell fails.
The same is true for Ring effects in an item...you can use the effect on a RING, but not on itself...
Generally we consider Ring spells to be a globe shape (like the rules mention under some of the spell descriptions.), above and below the ground...or whatever the surface....
That breaks the rules, since the ring duration specifically states that a magus has to follow the diameter of the ring when he casts it, even if it's already drawn. So at the very least, he'd need to lean down and follow it with his finger or walk along it if it's a particularly large hoop.
But, in the 'throwing-hoop' example, would the magus need to go around, tracing his finger, or is it ok if the hoop turns, and his finger is immobile? In both cases, his finger traces the circumference.
I think the mage needs to be the active element. Besides, it would not be easy to always toss a hoop such that made a perfect full circle in such a fashion.
That's what I'd say, but I'm just pouring fuel on the fire of this thread, in order to get around and dela with most potential abusive bugs.
Personally, I really think getting long durations (or almost permanency) from such spells falls outside the whole idea. Use magic devices for this.
I see very little chance for abuse since you have to be totally in the ring to be effected, and the ring doesn't extend great distances above and below. So magic finger rings won't work except for covering a knuckle or two. And you cant make light sabers and laser beams since a small ring doesn't have the range to be useful.
By throwing the hoop down you'd have perfect circle, which can easily be traced. In combat it can be difficult to complete a circle to the required degree of precision.
what would constitute breaking the circle.
for example a fire with ring/circle duration/target would be a real munchkin way of perpetual fire.
But would the smoke, soot or popping embers rising from the fire eventually break the circle cause the ring and spell to fail?
I seem to remember a canon covenant somewhere that uses creo ignem light generating rings as a cheap/easy way of providing magical light to their covenant.
I'm starting to convert to the 'should probably/usually be immobile' camp ... but really, some of the effects like 'preserve specimens' rings on a jar I like. To me it comes down to 'does it feel mythic, or gamist?'. If it ruins the feel of the game and is obviously an attempt to use the rules to one's advantage rather than play one's character then I of course don't like it. But if it actually has a mythic feel and 'makes sense'... well, I don't care if you carry twelve with you at all times.
Like say, twelve little metal discs inscribed with rings of light generation. Rather nondescript and easy to make, but easily a useful thing to have on you in the right situation. And not so very 'unmythic' I'd say?
I'd say no.
The fire, if a creo ignem effect, is just that. A perpetually magically created fire that would vanish without the circle. It doesn't need fuel, it doesn't sputter without an external reason, it doesn't throw off embers. I don't see how smoke could break the circle, unless the circle is inscribed on say a wall surface or some such and then it might gradually discolor the upper portion of the circle over time. Possibly a short ammount of time, depending on the size of the fire created.
And I don't see that as terribly munchkin? That is just a straight up use of two arts at a given range/target exactly by the core magic rules. Now if they tried to put said ring on a hoop and throw it over someone to get a continual source of damage or something... OK, Cheese. But inscribing one in the kitchen so the cook doesn't need a wood fire? No problem. It's good to be a mage.
but would they be as mythic as litlle rings inscribed around the covenant that don't move. How about magi casting spells to rid rooms of darkness, magically summoned torches that disappear at sunrise?
Little light disks aren't bad, but they don't strike me as being quite as neat as what they replace.
You were nit picking there...I was giving the basics, not the entire thing. If you like, I can write the Tractaus on the process for you, and you can spend this winter reading it.... 
Seriously, the point I was getting at is: You toss down the hoop and CAST the spell on it....If you move it (the Ring)... 
The spell fails because the spell moved....
Which brings up a point....Magical candle sticks and such could be ruled to NOT function if they are moved..Ie They 'turn on' at sunrise...if you move them, the function would stop until sundown...when the device would turn on again....
I like that Ring-duration allows the magi to easily light-up the covenant, without expending precious raw-vis on magic items or so on; but I could certainly see that go, too - just spend on creating LEDs or make-do with spells or (gasp!) mundane lighting.
Another use not mentioned (I think) is the use of Rings to ward doorways, windowsills, and entire rooms (mentioned in Covenants). This, I think, is really useful mechanically as well - inventing effective magical doors/wards without this is... well, expensive. The Circle/Ring low-level, low-cost wards mean that lots of labs might have such tiny warding spells, making the place more magical. Magical traps and wards become a normal part of a laboratory, as the flavour text sort of implies - not relegated to the paranoid magi willing to spend expensive raw vis and lots of time to protect their lab.
Another use mentioned in canon is the use of Circle/Ring to bind ghosts and spirits into led mirrors (amongst the Tremere Ledworkers). Seems perfectly cool and Mything to me, and I'm not sure how to do that without the Ring duration.
Do you want to be able to cast Circle spells on boats? I can understand the desire, if you expect lots of boat-travel in the saga. I'd prefer to drop the exception, though - it just seems incosistent, and a "boat" isn't specified, as noted above. I'd simply say that a Ring must remain immobile; if it is moved, it is broken.
I would insist that a Ring/Circle can only affect what is inside it AT THE TIME OF THE CASTING. Once the target leaves the ring. the spell is broken - hence, the target must be IN the ring for the spell to affect it to begin with. (Wards working differently - but they always are weird that way.) Arguably, this prevents effects such as CrIg light sources, as the created light is not within the ring when the spell is cast (only afterwards).
I'm undecided about the ring house rule. I don't think i'd adopt it. But just to give you something else to think about....
CrAu to create a storm, conc duration.
Then Maintaining the demanding spell, storm duration
Then cast anything else you want, storm duration
Relies on having the right virtue of course
(my book arrived this morning, this jumped out as obvious abuse)
unless it doesn't work, similar to CrAu a storm with storm duration.
Yes, LEDs are cool. And they make light, too. 
