Re: PCV for boats?
Near all new boats will have a PCV system. Typical older models just had one breather tube running up to the flame arrestor to suck/introduce the "fumes" back into the combustion chamber etc..
My 2003 equipped with a 350 Small block has a PCV valve on one valve cover (attached to a manifold vacuum source), then a vent tub on the other valve cover - going up to the flame arrestor as described above.
Cheers