I looked at a LOT of trailers that didn't have shower surrounds in the shower. And honestly it confuses me greatly. I agree that it just seems wrong to shower right up against the walls...
Anyway, if you add some kind of shower surround, you could add a layer of protection by painting about a 2' band of "red guard" around the bottom before you put in whatever poly sheeting you might use. Double protect by caulking in really well around the bottom, and all the seams, and I would think you'd have a pretty good system!
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!