Just thought of something while trying to map this island......
Rather then having invisible walls would it be possible to have
a script that can place the player back onto the island near the
point they fall or jump off? Basically a script that activates when
the player reaches a certain point below the island or a certain
horizontal distance?
If the island is not actually in water (say floating in mid air
in an interior cell set to act as an exterior), then when the
player falls past the last/bottom most object in the cell,
they are automatically placed back at the starting point
(no script needed). I took advantage of this in floating
mage island (with an island guide that explains this as a
"protective" measure, should the player fall over the edge).
If the islands are in water, though, then they would need
a script if the player swims (at least on the horizontal [one
could swim in that direction forever]). Swimming down,
though, would eventually have the same effect as the
previously mentioned method.