Oh, assuming that if I don't need to roll for spont effects then I also don't need to roll for weirdness factor?
Yes.
Scott
[OOC: I think it's base 4 rather than base 3 - base 3 is over a surface which can support your weight. That said, Level 15 is still within Fray's reach, and level 10 within Gregorius':
(Re 8 + Co 3 + Sta 1 + Aura 7 + die 4)/2 = 11.5, so works.
To cancel the spell: (Pe 0 + Vi 5 + Aura 7? + Sta 1+ W&G 1)/5 = 2.8, rounds to 3. Cancels out + 3 magnitudes + a stress die (no botch), so should work for a level 10 spell.]
Edited for misremembering Gregorius' Vim score, plus taking out the shouting.
You're dead right, sorry. I checked that too and still mucked it.
OK, so what happened exactly, and what are the magi doing?
Scott
I believe that the casting roll for both was still 30+ or greater before dividing for spont, so can still cast the same effects at the correct level of ReCo 15 and ReAn 15. So both effect still work. Alcimus and the grog should be gracefully floating across the surface of the water, where the effect gets cancelled by Fray at the other end.
Gregorius successfully cast his own effect to do the same, and cancelled it.
(edit): I'd assume that the Magi are waiting on the island for the others to get across, then starting to search the island in detail.
[I tried to post this late last night, but the board was down.]
The island turns out to be several dozen paces across. The terrain is rocky, but covered by soil in places, and, judging by the plant life, that soil is quite fertile. It's also somewhat hilly, and it's not possible to see the whole island from any one point.
Scott
Gregorius will cast another non-fatiguing spont to check the aura, then head to find the altar Fray mentioned previously.
Still at 7.
The altar is a simple stone, surrounded by no other structures. The symbols on it seem to indicate that it's dedicated to Poseidon. It's hard to say if it's been used recently--there are no ashes lying around.
Scott
Gregorius will have Alcimus look around to see if he can see any tracks or other indications of non-normal-animal presence. He'll also do another quick check of the aura next to the altar.
Once that's done, and assuming nothing remarkable is revealed, he'll explore the rest of the island, looking for any points of interest.
Alcimus doesn't find any trace of humans or anything else non-animal. The island doesn't seem to have any other remarkable features, but Gregorius, like Fray was earlier, is impressed by the quantity of life: the parts of hte island with soil are covered with plants, and, though the animals are mostly lnsects, lizards, and frogs, there's an impressive variety of each of those.
Scott
He'll wait to see if Fray has anything further he wants to do here.
As its his second time looking around I'm sure we'd have found anything untoward it it was easily apparent.
"Gregorius can you check for the presence of raw vis, as I have a feeling this place could provide Vis in some manner? It seems powerful enough."
Then .. "Do you wish to explore the other caverns straight away, or wish to wait here for a short period while I search the water?"
If they have no objections Fray will spont turning into a "fish".
- Effect level 25 = base 20, +1 diameter, to equivalent size "fish". I think a size 0 shark will be ok and move fast enough to take a short look around.
- CT: 53. Mu 21, Co/An 11x2 for focus, Sta 3, Aura 7.
- I'm interested in what is immediately around the island, and just how deep the cavern is. If I see anything remotely odd or dangerous I'll be heading back rather than confronting it.
Roll: 1d10=1, then 1d10=2. So a roll total of 57. - CT 28 vs Effect level 25. Weird check was ok too 1d10=7
As Fray saw from the air, there are quite a few fish. Further down, he also catches the shadow of something big moving in the deep.
Scott
Seeing that, Fray will return to the island.
I'm assuming that when you say big, you're meaning a lot larger than anyone should be comfortable seeing, with regard to fish sized. Could you put an order of magnitude to it?
"There is something large in the deeper water which makes me think we need to take care when crossing the water. While we don't have much of a choice for getting back to our entrance cave, we should limit our risks. I did not take a closer look, as I'd rather not upset it."
Big enough that fish-Fray would make a light snack for it.
Scott
OOC: I'm darn keen to look at what that is, but you've created just the right amount of "yikes !!!!!" for me to stay away for now. 
Fray will stay on the island assisting the others.
Gregorius will wait on the island whilst Fray goes swimming, checking for vis as suggested.
[Listen to detect raw vis, [strike]designed to be held as he wanders around[/strike] InVi Base 1 + 3 hearing = 4 vs (In 6 + Vi 5 + Sta 1 + Aura 7 + G 1)/5 = 4. He'll cast this repeatedly until he's covered the entire island (which I think shouldn't actually take that long given its size).]
Gregorius nods:
"Big enough that turning the water to ice around it isn't going to seriously impede it? That was my plan for problems whilst crossing."
"Once we're back off this island - and possibly just before we're planning to leave anyway - it might be possible to explore the lake by casting your Orb of Sunlight spell on a rock, and then Rego Terram-ing it around underwater with a Prying Eyes variant on it. Will probably need an Intangible Tunnel on it to maintain control over its movements."
"The creature seemed big enough to not even be slowed by that type of frozen effect. Far larger than any natural animal I can think of. Given the dedication of the monument, my bet is this is certainly a magical creature. If it shows while we are crossing I think we'll need to distract it while the others cross, and then quickly escape. It may not be worried by things outside the water though, so there is that."
Fray will Orb of Sunlight rocks when needed.
"The ice or glowing rocks might distract it, as long as it is not sentient."
As for vis...how do you figure the range of that spell?
Sorry for a the somewhat late reply--I was traveling and crashed another car, and just got in.
Scott