I've been asked to give a quote for furniture for a sailboat. The customer bought a sailboat built in 1973 and after starting to restore it they realized that they were in over their heads. I will actually see the boat tomorrow for the first time. They want me to quote on a cover for some electronics, a combination air conditioner cover and table and two cabinets. My question is this: they think this needs to be built with marine grade plywood, since its inside the boat is this really necessary? Not using marine grade ply will save them hundreds of dollars.

I've never built for a boat before but I can't imagine that its much different than my usual builds except that it will be, generally speaking, smaller and attached to the walls with glue.

Am I "missing the boat" (pun intended) or is it just a matter of finishing with a water resistant finish. The boat will be in the Great Lakes exclusively (never in salt water).