We had a similar crack in our fiberglass pool two years ago. It was also in the seat in the deep end. When we first discovered it, we tried to repair it with some epoxy. A month later, it started leaking again. Then I hired a local pool professional who opened up the crack some and stuck some butyl tape in there. That held for about 6 months before it started leaking again. I eventually had to get it permanently repaired with some fiberglass resin. I probably called 30 fiberglass pool companies in my state and bordering states before I finally found someone who was willing to travel to my house to permanently fix it with some fiberglass resin. I hate to say this but you really need to get that crack permanently repaired with some fiberglass resin ASAP because patching it with epoxy or any other material is only a temporary solution. If you let leaks go on too long, it can cause your pool to settle, which can cause additional leaks.