Hi,
Hey, I've seen this done on a diy show...
If the walls of the bathroom are dry wall or plaster board. you can use liquid nails to hold the cladding on. The clading used in bathroom was exterria clading for a house. So it was already sealed and ready to paint.
They did put sealer at the floor level and and top of the cladding to make sure no moisture got in underneath and /or from the top.. Looked fantastic, when finished.
Blick or concrete walls, a little trick... behind each of the boards. drill a 5 mm hole put in a plastic or timber plug. about every 400 mm.. Put screw / mail through cladding into plug/ timber dowl. This is allot of drilling and plugging. hard way of doing it if you have concrete or brick walls in bathroom. Or do battons about every 500mm..