I've pked many a retard in the water. Good strategy is to pull out the baton, beat them to near death. If they die even after being tasered and cuffed it's a pk. Do with that information as you will. If you stop moving while dragging them they sink. Accidents have been known to happen.
Remember the baton will not let you actually kill them don't be afraid to really let em have it. If they drown bc they're doing a retarded evasion strategy that's literally on them.