Like most things you leave outdoors, aboveground swimming pools will deteriorate with age and eventually spring leaks or develop cracks in the siding. Even if the cracks are beneath the waterline, repairing them can be fairly simple if you use hydraulic cement -- which begins curing upon contact with water -- to act as a stop-gap measure while you apply the appropriate patch for your type of pool. Be sure to drain the pool before attempting to patch a crack inside beneath the waterline.
Things You'll Need
Angle grinder with diamond blade
Hydraulic cement mix
Pool repair kit
Sand down the crack and the area around the crack with your sander.
Use your angle grinder to cut into the crack and extend it about a half-inch past where the crack currently stops.
Mix a small amount of hydraulic cement according to the directions on the container, then apply it to the crack to seal it up and prevent water from leaking out during the rest of the patching operation. Make sure that the cement goes into the crack itself, and not just over the top to get the best seal.
Follow the directions included with your repair kit to complete the pool patching procedure.
This guide can be used to patch cracks from both inside and outside the pool. If you choose to patch your pool from the inside, you will obviously need to drain the water before beginning the patching process. For information and advice on draining your pool, see the link included in the "Resource" section.
Do not attempt to use this method on pools with separate liners. These types of pools are not designed for use with hydraulic cement, and they will have their own methods of patching.