how far is the shower panel coming out past the curtain or doors? They have great bathroom paints. I have about 4" of tile outside my shower at the back end, so doesn't seem much water, and I don't enter and exit from that end. I have a properly sized exhaust fan so there is no condensation on the walls following steamy showers. Benjamin Moore paint has Aura Bath and Spa and its a very lovely flat finish.

It is a shower bath. More or less like this one: http://www.amazingbathroomsupplies.co.uk/images/plexicor-isede-shower-bath-1800-x-800mm-p23-15067_image.jpg

The walls in question are the two walls adjoined to the bath.

You definitely need panels or tile on those walls inside the shower. There are products that come in sheets for this purpose. Swanstone and others s.

Another possibility might be venetian plaster, which is mixed with wax and sealed. I've never used it myself, but my contractor is talking it up.