!'ve fixed serveral pools, one in January. Yuck!
It's rather simple. Order patches and under water glue online. Take a small sweeze bottle and fill with food coloring and water. Get a mask and take a look first, look for obvious holes. I carried some shiny coins to lay close to the holes when I found one so I could easily see it again. Find all holes first before trying to patch. You surface and tell a person at the pool edge the size of hole.They cut the patch and apply glue and fold the patch double with the glue inside, but don't crease it. That way the diver doesn't get glue everywhere and he can easily know which side is the glue side. Patch one hole at a time and then take your bottle of food coloring dye and squeeze a little around the patch and see if it is sucked into a leak. If it drifts away you have no leak.
I've fixed a dozen holes in one pool before, left by careless use of a pool bruch. To check for a drain leak, turn the pump off and put dye around the drain hole and see if it sucks it in. Lights are bad about leaking and are hard to fix. They must be caulked or the light may need replaced if it's old and rusted out. Holes are easy to fix.