Is it possible that the water is getting under the tiles from the edges or somewhere else? I'd hate to see you spend on sealing only to have the leak continue.
Whatever you choose, it will have to be re-done eventually. So, I suggest (respectfully) to pick something with minimal re-coat prep.
I'd be wary of coatings that go on top of the tiles & grout. They tend to peel up after the UV rays attack for a while. Re-coat prep is intensive.
Some are really slippery, too.
If you want a no-slip sealer that won't peel; non-silicone contaning outdoor rated penetrating sealer is a my suggestion.
Ultracare Penetrating Stone, Tile & Grout Sealer by Mapei is nice to work with. It has no silicone, so should you need to re-coat in the future any new product will adhere. Laticrete also makes a couple of very good sealers. Don't cheap-out on sealer.
Paraffin based sealers work, too but don't penetrate deeply.
If your tile is glazed (non-porous), there's no need for sealer on the tiles, just on the grout.
For glazed tiles & super-duper water proof forever grout, you can remove the old grout & use epoxy grout.
Paul
