I always thought that bodies of water were ground
Water (not distilled) is an OK conductor, but not a great one. If lightning hits in one part of a lake and is conducted to another, the voltage drop across (for instance) 1 meter of water will follow the classic $$V = iR $$. where R is the resistance along a 1 meter path. For drinking water, this is on the order of 20 to 200 ohms/meter, while sea water will have something like 0.2 ohms per meter. Let's strike a balance of about 10 ohms/meter for fresh water.

So how much current is found in a lightning strike? Generally, in the range of 5,000 to 50,000 amps. Obviously this will not be concentrated in a small volume of water, or the water would flash into steam, but 10s to hundreds of amps is a good first estimate. Keep in mind that when lightning hits sand the discharge channel can melt the sand to glass, called fulgurites. Even 10 amps through 200 ohms/meter will produce a voltage differential of 2,000 volts along the length of a  2-meter tall person in the water, and since this is obviously well-coupled to the skin (the person is IN the water, after all), it should be clear that there is lots of potential (pun intended) for problems.