I have educated myself about gfci's although I am a diy'er. My gfci has my dock plugged into it. About once a year it trips and can't be reset only replaced. All outlets on the dock are weather proof. I am trying to find out if I need to rewire the dock or if the supply to the gfci can cause the tripping. This is a critical issue I need to resolve as I understand the implications of swimming near the dock if electrified. Again, I'm concerned that if I replace the dock wiring and the supply to the gfci is causing the tripping then I have to replace the supply side, not an easy task.
I have found that GFIs installed outdoors do not last nearly as long as indoors. Your issue is likely the constant proximity to water.
My suggestion is to install the GFI at a point in the circuit inside the house, or even install a GFI breaker.