The key is to seal up the cracks in tile mortar joints, and then caulk the joint where it meets the tile on the wall.
There are other things you can do to waterproof the structure and prevent water from a leaky shower from seeping through without removing tiles. You need to seal everything on the outside and inside of the leaking shower. That includes all of your joints between walls and floor, any penetration through foundation wall (power outlet or plumbing), and anywhere tile meets mortar like a corner joint. Make sure it's sealed with silicone caulk. These are all areas that typically leak if they aren't sealed properly.
Grout will absorb water, and then once it's saturated it can become an almost sponge-like substance that can hold up to a cup of water in the same volume as itself. Water pressure can force water into the grout - and sometimes behind crossbars or tiles, where it can cause staining or mould. So you definitely want to seal up those joints with a silicone caulk designed for this purpose. This will deter water from coming in, but keep enough moisture (in the form of humidity) for your tiles to stay nice and clean.
You can if you make sure to use a specially designed sealer that works over tile surfaces. Just make sure your tile is clean and dry when you apply it, and that it's the right stuff - or else you'll just get more staining and mould issues (this happens often). Waterproofing material needs to be able to withstand water vapour from shower steam coming into contact with it for extended periods of time. Make sure if you want to waterproof over tile that your surface is completely clean and dry. Some people recommend using a damp towel to wipe up excess moisture from their tiles after every shower in order to prevent mould and mildew growth behind their tiles - this not only keeps water away so it doesn't seep into the grout or underneath crossbars but also stops any remaining moisture from sticking around long enough to cause problems. This would be an alternative method, but it's still important to let your tile dry thoroughly (without humidity) before applying a silicone caulk.
The bathroom is the second most common place for water to leak (through leaking showers) after a kitchen, so it's important that if your bathroom is leaking that you find and fix the problem as soon as possible. If you can't figure it out on your own, contact a professional for expert advice for a free quote. They can take care of the shower leak, inspect the wall and other surface, perform waterproofing on the surface in the bathrooms, repair, fill the gaps, repair any ceiling, paint, and do a lot more jobs. They offer complete quality work including waterproofing. Waterproofing includes repair work, determine where to apply silicon, and more. Quality waterproofing demands hard work and DIY isn't so efficient. The best way to waterproof a whole bathroom (including shower floor except floor waste or waste grate and shower walls) is to remove the existing tiles and start from scratch with new materials. This process involves applying a waterproofing membrane shower seal between the studs (or framing), sealing any penetrations through wall's foundation, stud walls, and installing a new pan (if surface-mounted plumbing has been leaking). If that's too much work - you may want to look into other options like silicone caulk. You can use floor grout for shower waterproofing.