Jeff, I'm sure there are different grades and I know there are different qualities. The real cheap
stuff tends to either go brittle or ooze uncured plasticizers, not good.
But I would expect that any padded flooring
material you can buy in rolls (i.e. the vinyls that replace linoleum) would be great for the job. You probably could buy a "roll end" from some flooring
distributor and get it heavily discounted, too.
I guess in theory a real marine
grade vinyl would have loads of arsenic or other anti-mold compounds in it. If there's any space in between the vinyl and the plywood
backer, that space will be a haven for mold
. You probably should glue the vinyl down 100% to the backer, so there's no space between the two materials, either adding a mildecide to the glue, or getting a non-water-based glue or one designed for damp places. There's got to be such a thing, mildew likes some glues more than others.