In a word, no. Many people think that the tile is a waterproofing layer...it is not. A properly built shower is waterproof (to the structure anyway) before any tile is installed. So, what you likely have (not uncommon in FL, and unfortunately elsewhere) is a shower built without any waterproof liner. Nothing you can put on top of that to make it waterproof while retaining the tile as a top layer...no sealer or topical waterproofing will keep moisture from migrating down, and getting to your walls again.
Time to tear it out all the way, repair what needs repairing, and rebuild using one of the approved, tested, methods (of which there is certainly more than one!). Personally, I like the Schluter shower system, www.schluter.com , but there are many other ways to build a shower that will work. The traditional shower pan is constructed with a preslope, a waterproof liner, a setting bed, then the tile. That liner creates a waterproof bowl that directs all of the water that gets to the pan to the drain. It sounds like what you have is a sloped pan with just tile on top, with no liner. A recipe for failure (but it may take awhile). In contrast, something using a topical waterproofing system (Schluter's is one), puts a totally waterproof layer on top of the shower walls and sloped pan, and then you install the tiles directly on top. There's much less that can ever get even damp, so especially in an area like FL, where its hot and humid for extended periods of time, I think is a better solution than a conventional shower with the liner and cbu walls. Any of the approved methods will create a long-lasting shower that won't damage the structure. check out www.johnbridge.com for some info and help on whatever type shower you decide on. You'll get help and review by many pros, where here, there's only one that thinks he's god and can do no wrong.