Well, that's a lot of useful information.
The swiftswim+waterbreathing combo is tempting, but I doubt it's feasible. Our speed and athletics are probably so bad that we still have no hope of outrunning slaughterfish, our everything else is so bad that we have no hope of fighting them, and as mentioned it might be harder to navigate like that.
Those invisibility scrolls sound really, really nice. A full minute? Assuming we've got anything else to help us move quickly with, 60 seconds should be more than enough time to get us out of any trouble we're likely to have on this journey.
...unless we get lost.
Anyway, scrolls seem to be the way to go. My recommendation is:
-Buy one of those invis scrolls.
-Inquire about the enchanter making a scroll for us. It might be a lot cheaper than producing a full magic item. Note that we should probably save our fancy, high-class paper for a better use, and just buy a sheet of normal stuff for a gold or whatever it goes for.
-If it isn't (which since we're already here probably means checking the price first, then checking the enchanter, then possibly coming back) find someone with a scroll of water walking and a potion of levitate.
Assuming we can actually find everything we need, that'd give us four minutes of water walking, levitation, and a near-guaranteed way out if we run into trouble. Cost will likely be most of what we have, however; invis is 120, water walking is worth 125 but the vendor will hate us (unless we can get it custom-enchanted for less, which makes at least some sense), and levitate should be under 50 but again hate.
There's a lot of other possible options we have, but we do want to deal with this relatively quickly, and I'm concerned not having the invis will result in us being eaten alive by ravenous cliff racers.
We're Mage's guild. They would kill us if they could get away with it.
Is there a rowboat we can rent? If all else fails, stand on a plank, row with a board.
This might be a better, cheaper option, but it won't save us from cliff racers and I suspect it'll turn out that nobody uses rowboats for the following reasons so there's none for sale or rent. Floating debris will probably be even scarcer.
...hey, you know what I just realized? If we knew any routes that took us
near Tel Fyr, we could theoretically book passage and then just jump ship. Wouldn't be perfect and I don't think it's feasible in this case (especially since I'm not sure there's any canon way to know which route boat captains are teleporting you over), but in theory it's there.
Actually, just checked a map. None of those destinations make sense to pass near Tel Fyr; in fact, no destinations at all do except Tel Uvirith, the player manor for if you join House Telvanni.
So, amusing thought, but not gonna happen unless we bribe the guy to "take the scenic route." Which, given that he doesn't like us...