Need waterproof ideas for wall area above a prefab tub surround...

You got this!! Its so important to prevent future problems, good luck!

You can apply a moisture resistant paint, preferably one that has an anti-fungal agent.

Exterior house paint works.

im sure some people will lol at this but I wanted something more than just paint to ensure that the wall would be protected from water when I had my hubs install a showerhead to our tub only main bathroom. He doubted my idea too until it was complete & now not just him but anyone who has seen it thinks it was genius! We didn't have the money for a insert tub surrounding but there is tile from the tub halfway to the ceiling so I bought a box of VINYL FLOOR SELF ADHESIVE SQUARES & a waterproof super griping adhesive to add to the wall to ensure the squares really stuck & also to smooth on the surface where each square met and also at the tile & ceiling lines. I bought white to mimic the look of a surrounding insert. This also covered any areas where my hubs had to cut into the wall while installing the shower & where he originally hang the shower curtain was to far out (the bar curves) & the vinyl squares covered those anchoring nail holes giving out new shower a sleek, easy to clean, & 100% waterproof surface!

You could also buy Rustoleum Marine Top Side paint. Very tough boat paint. $18 qt on amazon.