My guess on the seams showing would be, water is absorbing into plywood and swelling at the seam causing the distortion of the vinyl/coated decking surface. I'd pull the pontoon and allow to dry then apply a water proofing. Also consider sheeting the under-body with sheet aluminum to stop water saturation from your wake. This also will give you more mph and better fuel usage. a +++
Just a opinion![]()
Never a good idea to seal up the bottom side of the plywood and is also unnecessary. It needs to be able to breath and shed any moisture that permeates it. Otherwise it will hold the moisture and rot in short order. Now it you seal the entire sheet, with epoxy for example, then that would be fine..