Originally posted by jonnin:

... If there isn't anywhere else, it sometimes puts raid mobs inside or on your island no matter what you do.

Huh, never had that happen. You mean an island with nothing around? If the whole island is covered with base-items, nothing should be able to spawn there, I've done it several times. Only problems I had were with bigger zones and shallow water that was not covered; sometimes the game would spawn mobs in or above those areas. From what I know, on an island in the middle of the ocean with nothing else in sight, not even raids will spawn anything.
Edit: Of course the game might be bugging and doing stuff like that, but that's on the bug then.