the thrown rocks are a lesser damage but you still don't want them to wade out and toss rocks at your walls. Its best to be troll proofed (high enough terrain that they can't bash anything or throw a rock and hit anything other than terrain) as that will ALSO stop most other enemy like undead archers from shooting up your walls or whatever. Its also helpful if your island is near (about as far as you care to swim) other islands or mainland where things can spawn and sit there doing nothing. If there isn't anywhere else, it sometimes puts raid mobs inside or on your island no matter what you do.

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.

They don't swim so much as wade. Even in deep water their heads still protrude and they can still swing weapons and throw rocks. What the water does is slow them down but with the troll raid the first 2 that spawn are still there after the raid timer expires. One of them will eventually wander away and then despawn once out side render distance but frequently the 2nd one will stay until you kill it.

On another note: If the water at the edge of the base (if you terraformed it or build a wall around it) is still rather shallow, they will just stand there and smash through the terrain/walls and still do damage to anything built within that radius. Just keep it in mind, like if you build something at the outer edge of the island > might need repair after attack. If the island is big enough, maybe leave a radius around the actual base empty, functioning as a barrier.

Yep

Thankfully, they're pretty harmless in the water, IF it's deep enough.

if you can make the edge of the pool steep enough, they won't be able to climb out, though manipulating terrain near water can be rather challenging. I might recommend using terrain walls and/or steep trenches if you're high enough above the water level.

Further EDIT; since this is all related to the troll raid, you'll want to study up on how raid spawning behaves; when a raid occurs, it is important to know that the spawn radius of a raid is quite a bit larger than the actual raid area( the red circle on the map during a raid). as long as you can suppress mobs far enough out, in this case by being on an island, you should hopefully be able to spawn proof enough that you can at least guarantee they can only come from one direction.

Since you can still swim to and from the island I would wager you are still more than close enough for stuff to spawn on the mainland and try to get over to your island.