Sharks or barracudas ? (don't even thought about giant squids, let's be serious !!!)
Anyway, I also think (loud) that introduction of some limited animal life would greatly enhance the game, such as :
- tropical "ambiance" when at port would add a local color (parrots, music ?, etc....)
- seabirds during the day, not far away from coasts. They could even notify about proximity of a nearby seashore for players navigating in high sea.
- some whales, dolphins would also do it...
- sharks, especially when combat occurs (with a percentage of apparition for example)
- smaller fishes or turtles in shallow water would be nice....
My two cents.