Mark A Price said: I have been told by others that a GFCI is good for 80% of its rated value. Our magnetic starter is set at 16 amps and this particular GFCI Hubbell #GFSM120A is rated at 15 amps. So at 80% roughly is around 12 amps plus if it has tripped several times I am told that it could weaken it. I have ordered a new GFCI and it should arrive on Friday. I will keep you all posted if this fixes it.
Oh Kurt Lading get out of the shop from time to time and help your Superintendent out to solve some of his problems out on the course 
This is true. Same with breakers. But you really need to know what start up load is on the motor as it will be higher than run load.
A few things can trip a GFCI. 1) being a short in the water that is touching ground, 2) a bad capacitor in the motor, or 3) your motor drawing more amps than what the GFCI can handle. The motor could also be locked up which means more amp draw.
Either way if the local distributor can't figure it out, call Otterbine direct. At the very least they can put some heat on the distributor to get you fixed.