We have a leaky bow assembly and its not going to get fixed anytime soon. Water
run back would soak the foot of the v-berth mattress and make for some sleeping discomfort. So for our 6 week cruise
of Georgian Bay I put down some dimpled construction basement wrap and it worked like a charm. It goes by the trade
name of Platon. It worked like a charm and kept our mattress (feet) nice and dry the entire time.
I can't say it did anything to add extra padding comfort but it didn't make it any worse either. I cut it 2" wider along the sides and foot, and folded these up along the corresponding edges. I also cut it down the middle to give me some access to the lockers underneath. I like it enough that I'm going to make it more permanent and cut out locker openings and secure it better.
I hope this helps others in their searches for avoid a wet mattress.