I'm completely rebuilding the formerly wet bath in our 71 Tradewind; I plan to make the shower pan using 1/4" plywood w/ epoxy and stitch & glue marine construction. This has worked very well for small boats and a shower to me is just an inverted boat
. The wall were already largely waterproof; they'll get caulked, of course. There will be a shower curtain with a waist-high "keeper" rail on the inside to keep the water off the sections of wall that are merely water resistant (.e.g window, etc). I decided to run the water supply plumbing (PEX, red & blue) in the open.