I am not sure that I fully understand the question. Is the question referring to a power system (<=480V) ground wire, a ground system, or a ground grid. I will try to briefly answer all three:
Electrical grounding on low voltage pool systems is tested the same way as it is in any other application. Several methods exist and there are even further varieties within each type including: Power system (green wire), bonding (interconnections between equipment), grounding (interconnections between bonded equipment and the ground grid/rod), and ground grid test (testing of the copper ground mesh).
Most commercial testers offer a variety of a point to point check (2 wire), all the way to a detailed ground grid test. You would want to get one that (at least) offers capabilities to perform the Wenner method or Schmumberger method (these allow you to test the dirt / soil / mother earth itself - soil ersistivity).
Water treatment plants and facilities generally contain corrosive agents such as caustic, ozone, and chlorine. These tend to corrode / build up on metallic parts such as bolts, washers, nuts etc. and result in a higher impedance ground network - not good for the network, even worst for the worker. Grounding and bonding tests identifies these areas and allows for the planning and restoration of a good grounding system.
Power system (green wire) test is a bonding test between the wire and the equipment enclosure and a second test between the green wire and its source.
One would NEVER attempt to directly ground the water or water system in a pool.