I know of a bath that was covered in T&G light coloured by the owners themselves. Looks lovely. A handy man could it. They matched the toilet seat to it.
Sandals is spot on. Any builders providers will sell t&g panelling required to do this. Just put in a simple timber frame to attach it to. Straightforward DIY job if you're any way handy.
A varnish finish will give it all the protection it needs to handle the regular splahses for years. Use marine varnish if you're explecting a lot of splashing.
I did that in my bathroom years ago, ordinary tongue & groove boards from Woodies, I just painted it with satinwood paint, it hasn't had any damage from water over the years, put it along the side of the bath too up to dado rail height kind of. No tiles, don't like tiling, not a nice diy job.