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 (they dip in fairly deep).
If you try to swim onto them, you can't - you are unable to get out of the water. Once I also got stuck, totally unable to move.
(Tested in Editor, default date and weather, early morning - about 5am I think).