Mix up a batch of swimming pool thinset repair mortar. Follow the instructions on the packaging to measure and mix the mortar and water. Use a trowel to stir the mixture in a plastic container or metal bowl. Keep adjusting the proportions as needed until the mixture reaches a spreadable, peanut butter-like consistency. Stir well to get rid of any lumps.

The thick, spreadable consistency will help keep the mortar from dripping into the pool water. Some thinset mortars also come with a bonding agent, which increases the adhesive quality. Mix equal parts mortar and bonding agent, then add water until the mixture thickens to a creamy, spreadable texture.

Spread a 1⁄8 inch (0.32 cm) layer of thinset on the back of the new pool tile. With your trowel, scoop up a daub of thinset and smooth it out over the entire back of the swimming pool tile, all the way up to the edges. Line the tile up on the pool wall so it matches any existing pattern, then press it firmly against the beam. Thinset dries quickly, so make sure to place the marble tile immediately after covering the back.

Keep the marble tiles in place while they dry for 24-48 hours. Check on the tiles’ progress every 10 minutes for 1-2 hours to see if they’ve slipped down the pool wall. If any small tiles slip, simply slide them back up and press them into place. For larger onliy tiles, you should duct tape them into place while they dry. 24 hours should be enough to dry the thinset mortar, but waiting up to 48 hours will let it cure and harden even more.

Use waterproof mosaic tile grout to fill in the spaces between the wood tiles. Load up a flexible spreader with grout and push the product between the tiles. Once you’ve filled up the space, let the grout set for 10-15 minutes, then use wet your hands and gently rub at any excess grout smeared on the tiles.

To avoid dropping any daubs of grout into the pool, work with small amounts at a time. If the dried-on grout is stubborn, wait 24 hours and scrub the tiles with a stiff brush to polish and remove any haze. After letting the grout dry for 24 hours, you can refill the pool and enjoy the product of your hard work.