There should be something that the tile is laid on such as cement board. It might be a problem removing the old grout. Later, if the tile is the right type and installed properly, there should be no water problem. My daughter and SIL bought a nice house a few years ago. My daughter was getting something out of the back of the closet next to the shower in the MBR. The wall was covered in mold as was the crawl space. A tile installer looked at it and determined that tile was not made for shower or water environments. It cost a chunk of change to have it replaces and cleaned up.
A diamond is how coal reacts under pressure.