Is your finish on the pool white plaster? I just used a product called Perma-Pool-Patch to reset a bunch of tiles that had fallen off our pool. It is really for repairing the plaster, but the pool store also recommended it for resetting tiles and even as a white grout. It was fairly easy to use, I used a plastic cup with a plastic spoon and plastic knife, and could use about three spoonfuls of powder to a slight bit of water, mix, and apply to 3-4 tiles at a time before it became too set up to use. It sets up FAST.
It can be used underwater, too, it needs to be mixed thicker, more like a putty, rolled in your hand like play-doh. I reset about 9 tiles underwater on the steps and it worked great as long as I had it mixed thickly - when it was too thin, the tiles popped off. However, I couldn't reset the wall tiles underwater, they kept falling off and I may not have been getting the consistency correct. I eventually drained the water to just below the tiles and set them all. It's only been about four days, but they seem rock solid right now.
Good luck!
