What would the substrate be? Marine paint, like Awlgrip, is epoxy. (It is sooooooo nasty to work with. Make sure you have a serious, serious respirator.) But marine paint is usually applied over either epoxy resin-coated wood or fiberglass, making a pretty waterproof surface even before painting. I don't think I'd be comfortable just painting it on drywall. What were you thinking of painting it on?

If you're going to build some type of structure to paint, with carefully sealed corners and all that, you could build in niches for shampoo just like a tiled enclosure, I guess.

Also, remember that marine paint is super glossy, not matte like regular wall paint. So your room style would need to reflect that.

I think someone here (Pharoah?) has a shower that they lined in mahogany and coated with marine varnish. That's basically the same idea, so they might have some tips.