In Tuvanaka, Tanoa, at coordinates 015 119, at the northernmost of the three piers, there are stone steps dipping into the water. You are supposed to be able to swim onto these to access the pier.
If you swim onto them, you can't climb on them, and I got stuck, totally unable to move, one time.
(Tested in Editor, default date and weather, early morning - about 5am I think).