The island isn't built yet... so it depends^^Not too many options for above-ground animals. Frogs are the obvious choice, but I would not do that with Poly's. It also needs to be warm water, so that doesn't give much breathing room either. How big will the island be?
Depnding on the wood and stones i can get. May it will be just a shallow with some wood rising out of the water.
And i think there is always the possibility to enlarge the island or add some floating wood / a turtle island.