Re: Bill
Hi welcome to the forum.
If it's repairing a tile or two in a swimming pool. You'd need to use diving equipment to go down and clear out the tile and adhesive, then back-butter the tile out of the water, place it into a plastic bag, go down and quickly rip off the back and place it into place, use the adhesive to grout with by way of allowing it to ooze out a bit and use your finger or whatever to smooth it off.
You will need some form of water hoover (sorry I'm not a techie in this but do know such a thing exists - perhaps for hire?) when both getting up the old tile, and also when pushing the tile into place to catch any adhesive floating off.
Adhesive will cure under water just fine. You might need to check with adhesive manufacturers with regards to whether chemicals in the pool will react to chemicals in the adhesive or visa versa.
Never seen it done but heard of quite few over the years doing it so it is possible and just fiddly more than complicated.