1) Moving meeple (in water) onto a boat on the same sea space: Is it swimming?
2) Monster tiles: Can a Serpent(for example) teleport out of the middle sea space that is surrounded by island tiles?
3) Monster tiles: Does the monster have to finish it's move on a completely empty space?
1) Yes it is swimming. (S)he swims to the boat before climbing in. You can therefore move the boat and then disembark to dry land if you wish (and haven't used any movement points first).
(1 swim, 1 normal, 1 normal).
2) Yes.. rules say any vacant tiles.
3) see (2) above.