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.