Think the answer is to use a so called waterproof adhesive (suitable for
showers etc - not swimming pools ;-)) - usually the next more expensive
one in the range.

I would go for BAL White Star in this application. I am worried by the
undercoat, though. A solvent based undercoat could possibly reduce adhesion.
I would, at least, rub it down briskly with coarse sandpaper first.

Christian.