Is this a tiled shower?
Basically, anything you do topically to a tiled shower is a bandaid, something deeper is wrong. A conventional tiled shower should have a waterproof liner up the wall at least a couple inches above the top of the curb, so even if it was filled to the brim so to speak, it shouldn't leak out. There are lots of ways to do this wrong, though.
The tile and grout are not waterproof, only resistant. It is what's underneath that matters.
Describe the construction - prefabricated pan, tiled, etc. and maybe we can get a better feel for this. Also, highly recommend www.johnbridge.com for tiling and showers.