You were wandering around at night in your boat, there was rumours of a creature scaring fishes & sailors off therefore many were forbidden from entering those waters at midnight, your curiosity got the better of you as you snuck out

For a while you saw nothing and as you were about to return back to shore, though a low wave pushed you further out to see as a light thud of a tail-fin hitting u further out to sea before it started circulating your boat splashing water with its tail fin at u