Leaking showers and baths are caused by:
By a number of factors that are hidden from sight, such as leaking pipes in the wall or a leaking outlet. The moisture is trapped and there is no outlet to allow drying out.
Evidence of a leaking shower and bath:
Paint flaking and bubbling at the base of the wall.
Discolouration of the paint.
Efflorescent salts visible on the surface, usually sulphates.
Treating the leaking shower and bath:
The tiling in the shower needs to be removed.
The base of the shower / walls need to be re-grouted with a waterproofing grouting and then re-tiled.
The bath needs to be removed to find and fix the water ingress problem, and then only can you repair the damp problem in a systematic way.