Just had a look at that - cracking trip!
Instead of trying to seal it have you thought of feeding air in?? If you have on-board air, how about feeding off into underseat box with a switch. Seal it up as best as you can and if you are going to wade, flick switch before hand. Air leaking out will stop water coming in. Not sure what pressure you would need to run it at to work but I wouldn't think it was much.
Or could be a completely stupid idea!
Steve