You will not be able to make this circuit as safe as a GFCI circuit, due to the voltage, current, and the fact it is a three-phase system. GFCI circuits trip at 4-6 mA and therefore ensure safety of people. A ground-fault protection scheme for this circuit will likely have a minimum trip level of a few hundred milliamps, depending on how sensitive a ground fault relay you can find and the charging current of the circuit. A 600 foot cable in water will probably have a high charging current. The trip setting must be higher than the charging current. A ground-fault protection scheme in this situation will reduce the hazard to people, but will not ensure their safety in a shock situation.
My recommendation would be a separate ground fault relay with a window-type (core balance, zero sequence) current transformer surrounding all three phase conductors. This relay should feed a circuit breaker with a UV release; this will make the circuit fail-safe as opposed to using a shunt trip.
The other option is to use an electronic circuit breaker with internal current sensors. However, these breakers use residual sensing to determine ground fault current (not quite as accurate as the window CT) and tend to have minimum settings in the ampere, not milliampere, range.
These schemes will work only if the pump is fed from a wye secondary system, not a delta system. It may also be advantageous to resistance-ground the neutral to limit ground fault current to 5 A or less and improve the sensitivity of the ground-fault detection.
Either solution will be quite expensive, certainly several hundred dollars, probably more than $1000. Perhaps there are some manufacturers of pump protection panels that incorporate ground fault detection that could help you out; I seem to recall seeing these panels on the market a few years ago for submersible and swimming pool pumps.
