Can't tell for sure, but that looks like it might be a PacFab Hatteras fixture. If so, those are notorious for melting the potting compound at the back of the fixture, around the bulb socket. It melts into a sticky goo and runs to the front of the fixture. Eventually, the fixture starts to overheat and blow bulbs and/or leak from the rear.
My pool (built in 1996) still has two of these fixtures, but I'll replace both of them as soon as the bulbs fail. I've already replaced one due to the melting compound issue. A standard Amerilite fit perfectly in the niche.
Don't put any money into the fixture, replace it properly by pulling the wire. Underwater splice should only be attempted as a last resort if you can't get the cable through the conduit and ONLY if the lighting circuit is converted to 12v.