There are dozens of waterproofing products out there and debates on what works best to follow; however, in Jeff's experience, he has had the best results with Schluter systems Kerdi membrane. At the same time, Jeff argues that the waterproofing component of the membrane key, rather than rely on the tile/grout to be the waterproofing factor. Remember that grout is porous and will draw in moisture between the tile. Waterproof it right! At the same time make sure you use the proper cement.