ahhha septic huh? well I would do an external drain to the outside. I wouldn't put saltwater down the septic, I would drain onto the ground outside, maybe have the pump discharge out there.
Just a thought, I don't know if it would be too much work, it's just that septics are touchy (and Expen$$ive) so i don't know if i'd want to plumb it into the sewer.
As far as wafer board what is that? the stuff they put under siding on a house? or the subfloor? I'd think it would work, as long as its painted with a good waterproof type.