The big advantage to marine grade ply without voids is it bends well. Since with fuselage construction the frames are all dead flat without bends it's primary advantage is lost. All plywood should have the end plies sealed, even aircraft grade which is far better than marine grade. Since 99% of the plywood now days has exterior glue that isn't an issue either.

The big problem with wood of any kind in water, plywood too, is getting wet and staying wet with no chance of it getting dry between uses. On boats that stay in the water that's a valid concern, kayaks though generally are stored dry and any SOF should be well aired out and allowed to dry between uses anyway so that's not an issue either.

My first Dacron/glass kayak is now around 5 or 6 years old, looks just like it did when it was completed and the cheap big box birch plywood which really was awful has held up fine, but it's sealed in epoxy.

Bill H.