It's hard to beat CCA Treated, sheathing grade plywood for damp situations.
It is very resistant to rot....AND cheap. However, since it is treated with
copper arsenate, you'll want to seal it on the interior if you use carpet
rather than vinyl. They're supposed to quit making it soon because of the
environmental problems with the treatment. It's still available around here,
though and they say they're coming out with a more environmentally
acceptable treatment that is "just as good". You might ask at your
lumberyard.
Beekman