Once upon a time the S61s belonging to Bristow Helicopters practised landings on Corby Loch as a matter of course. Then the CAA made them put anti-collison lights on the lower hull so they were not waterproof any more. Rather then go through the hassle of taping it over they stopped the exercise.
Judging by the amount of waterproofing tape on the Duke's aircraft I would imagine that if it landed on the sea for real the cabin would be awash in about three minutes.