This problem is very difficult to put a number on!
It is simply how fast will the camera hit the water!
But how fast is dependent on alot which makes it difficult.
Because the boat will hit the water first and the water will exert a reaction on the boat due to the volume of air etc...So technique has alot to do with the forces exerted on the seal by the water..Also depending on where the camera is located...boat is easier to make assumptions about, person very difficult!
The landing after drops is always less dense due to the churning water where as seals I believe are rated in calm water.
Water type and temp will also play a role.
Beause its very interesting and I know you probably want a number on it I'll give you a very VERY simplified version.
Just dropping a camera off a diving board assuming the camera is rated at 10ft = approx 85 N/sq in
Assume the seal breakage occurs immediately after impact..(.01s) and the mass of my camera, then a drop of nearly 1 meter would elicit an impact force equal to that which would penetrate the seal in calm water.
Disclaimer::Any other engineers out there don't give me crap for me EXTREME over simplification of the problem! Just having fun!