All GFCI breakers require that the HOT/NEUTRAL/GROUND configuration be properly wired or they won't work correctly. This is because they detect current greater than a milliamp or two flowing in the ground lead (a bad thing). If you have older residential wiring that does not use a dedicated ground lead or at least use metal conduit (properly configured as ground) then you cannot install a GFCI outlet or breaker without upgrading the wiring.

This may be slightly oversimplified---a GFCI which monitored the ground lead would be oblivious to current flowing from the hot lead to earth ground (cold water pipe?) through someone's body.

The GFCIs that I have installed require only hot ("line" in electrical terminology) and neutral leads. The neutral is which is where the return current from the hot lead is supposed to flow. The circuitry compares what is going out (hot lead) and what is coming back on the neutral, and if the difference exceeds about six milliamperes, the hot lead is opened, shutting off the power. It doesn't matter whether the leakage is to the grounding conductor or to "earth", and in fact GFCIs work in ungrounded systems. What is important is that all current return on the neutral, instead of through some other path, and failure of this condition is what the GFCI detects.

The purpose of the ground is two-fold: it keeps things that are supposed to be non-energized (such as the housing of an appliance or tool) from going "hot" if there is an internal fault, and it causes such a fault to draw enough current to immediately trip the circuit breaker. If you have anything electrical in use, near water or otherwise, you really should have grounded wiring, and the next best thing is GFCI protection. The very best thing, of course, is to have both.