Drain the water from the pool, or at least to a depth well below the tile needing replacement. Many tile problems are located at the top of the pool, above the liner, but for masonry pools with tile surfaces under water, total draining is necessary to replace lower level tiles.
Remove all loose tiles and chip off residual grout or glue from the base. You need a smooth, clean area. Use the hammer and file if grout sticks, by placing the file against the residue and tapping carefully.
Allow the area to dry for several days. The base surface must be dry for the new grout and glue to hold efficiently. Check at this time for any base cracks that need to be filled with masonry (if you have that kind of pool), and repair.
Apply the marine adhesive with a putty knife in a thin layer and carefully fit the tiles into place, making sure to leave uniform gaps between the tiles for filling with grout or caulking later. Allow the tiles to set for time recommended on the adhesive container.
Fill the cracks with epoxy grout or marine caulking. If you're using grout, expect some to fall into the pool. Consider having an assistant stand with a container to catch the spills. Smooth the grout or caulking, making sure it fills all gaps securely. Wipe excess off the surface of the tile before it sets.