I've used the dive well to and from Tuatha's Garden without problem several times, but I do know that it has some things cluttered around it, including some bug spawning points, so my best guess is that it's a bug....either a programing bug, or a bug that spawned next to the well blocking your return.
If it's blocked, the only ways I know to get off the island would be
1) Escape Dust if you've bought it from the special merchant already (visa level 2)
2) Zeneport if you have martial arts skill up to level 5.
3) Return home using your cell phone.
4) Teleport using cheats.
5) Teleport using a teleportation pad at a vacation home already purchased in France.
I generally purchase some escape dust as soon as I can, even if I almost never use it. I'd rather carry it around and never need it then need it and not have it.