I've been dealing with a leak at one of my returns for two years now; mostly kept it in check. It finally got so bad that I was losing 2.5" of water everyday this year after my niece picked apart my previous repair work. My local pool store is good, he's a builder. Kept telling me to go get GE silicone#2 and just seal it up until the permanent solution of breaking up the cool-deck concrete and replacing the return.
This stuff is great. It cures underwater. I couldn't believe it would seal the leak I had that was visibly 3mm by 20mm, a huge gash by any means. But it did.
So if you have a leak, try this first. It's pliable, flexible and most importantly - removable unlike epoxy.
Finding the leak is another thing. Food coloring doesn't match a syringe with leak detection dye.
Never, ever, ever use plumbers putty in a pool or PVC. It breaks down the PVC or plastic. I learned my lesson.