I doubt capacitance has anything to do with it. Reactive circuits can cause current to lag behind voltage but the total current in the circuit is equal at any point in time. (according to Mr Kirchoff). The amount going in always equals the amounty coming out right then. The GFCI just senses that it is not all coming out through the circuit conductors. That is a leak somewhere.
Bob, I doubt marine grade will really help you that much on the GFCI problem. That really just means the metals are less corrosive (usually stainless).
Maybe if it was NEMA 6R?
