Often there are outlet(s) required to be within a certain distance from waters edge. Scary huh? There is a reason for this. Those outlets also need to be gfi protected. The reason they are required in some areas is, if you wanted to have say a radio at the pool, you would use this protected outlet instead of running an extension cord all the way from your shop, where that outlet may not be protected. I just explained all that because that gfi circuit would probably have the pool/spa lights on it as well. You mentioned that you were having problems with that gfi "phantom" tripping, but has seemed to resolve itself. It could be that, if you had outlets around the pool on the same circuit as the light, that one of the outlets may have been causing your trips. Perhaps the outlet, while protected against rain or moisture (with a rain tight cover) was still getting enough moisture intrusion to trip. This could be from a cover not quite covering the outlet or something plugged in, leaving the cover open.
Also, every outlet, light, or what ever is connected to the gfi circuit needs to have its neutral return to the gfi breaker or gfi outlet, not the neutral bus in the panel, but directly to the breaker/ outlet. The breaker or outlet has its own neutral terminal that connects to the panels neutral buss. If you have a component of the circuit who's neutral is not returning to the gfi, That too could be your intermittent or phantom trip. This is worth checking out because the component whos neutral is not returning to the gfi's neutral terminal wont be gfi protected even though the gfi sets or re-sets.
Does that make sense?