I'm rather tempted to introduce you to a very advanced concept, a rowing boat
If a mundane is to activate a magical effect which required no Vis, I can off the top of my head think of Charged Items and some of those dreadful Mercere Mutantes tricks that I want purified with fire.
The simplest solution to your puzzle (since it does not indicate a requirement for a use of magic) is to use an entrely mundane method of crossng the water, which is something too many of us tend to forget on this board.
Rowing boat works.
Auto-creating bridge items works
Hermes portal works
Short teleport (up to 500 paces) item works
Might creature transporting you through the water works
An enchanted tree throwing you through the air works, but the landing might be rough.
Close. There actually used to be a natural island (now submerged) near Cherry Island, about a kilometer NNE (as the swan flies) of Fort Augustus, by the name of Dog Island. Hence the covenant name of Insula Canaria. I have found next to no information about the island (although I did find a .pdf account of a parson who did some underwater exploration around Cherry Island in 1908, who speculates that Dog Island was about 15x20 yards, which is a little smaller than I thought it was, but I can handwave it to be a little larger). And that's basically more than I've found in a couple of months of poking around, beyond that it was there and submerged when they built the Caledonian Canal in the early 1800s and raised the loch level by about two meters.
No. I want to achieve this end using magic. Specifically because this island, being on the one spot of Magic: 6 aura in the area, is where they've built their main lab. Thus, it needs to be accessible only by those with the "password." And the reason I'm wanting a bridge instead of a Charon's Skiff or a Portkey or a Stargate is so that they can, if needed, take a cart with laboratory items or supplies to the island as well.
This is in no way opposed to what I am saying. Using magic. But only formulaic, or, better, spontaneous spells. "Password protected". Fully automated (no need of grogs at either end). And usable by mundanes who have no magic of their own. A "bridge" in the sense that it must allow a mule with a cart to "walk" all the way from the island and back. Suits you?
The ol' gal is actually their primary remaining vis source. Her eggs are Animal vis. My ex Misc Daughter of Circe who lived there the last twenty-five years used to brew potions (Draught of Frigid Tolerance and Essence of Gillweed) to enable them to fetch the eggs from one of her nesting sites.
Not that I've been thinking about this way too much the last six months or so.
I still think keying it off casting privileges is an elegant solution, indeed it works better with both ends within the aegis because the user is interacting with the aegis the same way at either end and so "feel" the same to the InVi effect. There is nothing stopping you giving a trusted mundane a casting token.
Create two posts as enchanted item: CrHe to create bridge between them up to X in size. Item maintains concentration until cancelled. In the lab they would be a few feet apart, when pounded into the ground at edge of the island they are 450' apart.
Activation: Have indentation on both sides of the item for some sort of device to be inserted. Insert a special key carved to the right shape into one indentation, it creates bridge, insert into the other one and it stops. Whoever is authorized to use the lab for the season gets one of the keys to activate and deactivate it.
Effect 1: InHe (or InTe or whatever) to detect if item touched to its side is proper key
Effect 2: CHe Base 3 (likethe wall of living wood) +1 concentration,+1 touch, + +5 size (you need supports into water) so level 30 +5 item maintaince concentration + 10 for unlimit uses = level 45 item.
Enchant a single tree trunk trimmed to 10 feet in length (that will easily fit in most labs) and about a foot in diameter. Enchant it and as part of enchanting processing, split it evenly in half to make two posts 5' long each of which will be sunk a foot or two into the ground on the shore.
By strict RAW you can't. That's the whole point of the discussion and the reason why this thread moved past 3 posts.
Hermetic Architecture (TMRE p.97ff) keeps being mentioned because it explicitly allows split items (of sorts).
No, you get a nce piece of (very expensive, vis-wise) wood.
You probably couldn't even argue (valdly) that each part had been opened for enchantment as each might well end up with too little Vim Vis in it for that.
Your troupe may rule that this works nd that's perfectly fine, but is not RAW (without a relevant virtue).
Have a look at Hermetic Projects, box on p. 114, and/or possibly Reforging Enchanted Items, HoH: MC p. 125ff.
SO enchant one post and make sure the other is arcane connection to it (fixed) and the inhe effects is arcane connection range for the key being rubbed against it or the one it is connected to.
Of course you could just have a CrHe linked to each post and PeVi (unraveling form of Herbam) linked to each one. Two enchanted items so doesn't matter which one createst the bridge, the other could dispel it.