I had one of two new bulb installations with water creeping in after a couple of weeks. Drove me crazy removing and refitting the gasket and locking ring. Finally went to using a coat of silicon around the outside of the gasket, which worked for me.
As an aside, commonly misunderstood is the degree to which silicon will degrade in water, salt or otherwise. Fresh and salt water aquarians builders make extensive use of silicon in aquarium builds, both for its superior water tight and strengthing properties; and generally its resistant to degradation from uv light due to their stable Si-O bonds (chains of repeating silicon-oxygen bonds, or Si-O). Not all silicons are alike however so it’s important to choose the right one. Loctite Clear Silicon performed exceptionally well for me if you choose to go that route (and favored by aquarium enthusiasts who build their own tanks).
FWIW, I found my one bulb replacement vexing, to say the least. It had enough cord to allow me to submerge it under water in my adjacent spill over spa so I could monitor it (weighted down as it will float). A learned lesson from having put back my first replaced bulb into the pool wall niche only to discover a water line in it 4 days later (hence the wisdom mentioned above of monitoring . . . to be safe, for at least a week as leakage can take quite a while to manifest).
Best of luck.
