I think part of the answer depends on how you're building the wall. Another part depends on how thick the track for the sliding door is.
The sliding door track will determine the thickness of the wall. It needs to be able to fit in between the skins of the wall. I would think that if you're making the skins out of 1/4" plywood, that should provide enough support for the track and door. May want to test that theory first though, and add additional support if needed, like maybe using 1/2" ply or adding framing inside the wall to support the track. All depends on how heavy the track and door is. If the door sits in a track on the floor and doesn't just hang from the overhead track, then you have much less weight to support.
If you look here at posts 268 & 269 (http://www.airforums.com/forums/f185...-50967-20.html
), you'll see how we're building the wall between the bath and bedroom. Maybe that will give you some ideas.
If the wet side of the wall is plywood that you will cover with something waterproof (which is what we're doing), then I think the plywood or frame part of the wall should go to the floor. That will help keep the majority of the wall weight off the lip of the tub. Then the covering (fiberglass panels in our case) will attach to the plywood and overlap the lip on the tub. It'll be caulked well to prevent water leaking in behind the fiberglass.
If the wet wall is just a waterproof panel, then the panel should go over the lip of the tub so you can seal it properly.
Hope that helps.