3. Water- since it is a GFI
4. Faulty breaker
If the breaker is indeed tripping, all components in the circuit should shut down. Since the light remains on I would guess that current is leaking through the breaker. Is the light as bright as it should be, or is it dimmer? You can verify all of the above with test equipment. If you are unfamiliar with testing procedures then you would have no choice but to call an electrician. Hope this helps.