There really should be some sort of metal or membrane roofing below the tile and tile bed, as tile floors are not 100% waterproof. I don't think that sealing the tile or grout is really going to solve your problem. If you are lucky, the underlying roof is still intact and your problem is coming from a different source. The roof is probably slightly pitched to one direction or another. Maybe some whater is sneaking in at the sides or at the uphill end where there is flashing, or gutter, or downspout etc. If you can't find an obvious source like this then you might end up needing to bring in a pro who will either find the leak or tell you to tear up and replace the whole deal. If this ends up being the case, it may be cheaper to just put a roof over it and make it into a Florida room or a three season porch.