I have / had a small underground suction side leak between my skimmer and my pump that has needed attention for some time, the pool would only loose water when the pump was off, and a little air would get into the pump while it was running. Well lately this small leak has been turning into a medium sized leak, and before breaking down and jack hammering up the concrete I decided to try this magic in a bottle product. I don't know if it will work yet, but here is what I have done so far, according to the directions for this product when using in suction lines you must disconnect the pump and replumb them so they can be pressurized. This product can also clog pool filters so care must be taken when using.
First I disconnected the pool pump and capped off the suction side pipe stub, then I drained the pool down to about 2 inches below the skimmer using an engine powered water pump, once the water was below the skimmer I could see the leak was more serious than I had thought, as the water in the skimmer basket was dropping by about an inch per minute. Once the pool water level was safely below the skimmer I added about 1/3 of the bottle of the Mar-Lig Fix a leak to the skimmer, and pressurized the line to force the liquid through the leak using a small submersible pump to draw water from the pool and pump it into a garden hose to which I attached a home made PVC pipe / rubber hose attachment which fit into the skimmer vacuum port. After about 5 minutes I removed the hose as the seal was not as good as I would like and the skimmer basket holder was starting to fill up with water. I was somewhat surprised to find that this water was not dropping as fast as it was earlier so I let it sit for a few minutes. 15-20 minutes later the water had only dropped about an inch, so I added some more water filling the basket holder up just in case the leak was in the skimmer housing, an hour later this level had only dropped by about 1/8 of an inch, and held there, we will see what it looks like in the morning. If all is still looking good I will wait about 48 hours (per the instructions) before flushing out the excess and reconnecting the pump. So only time will tell if this stuff really does work, but it is certainly cheap enough to try given the alternative.