Would you consider it unfair if you were to take the time to photograph, and sketch out the landscape every time the water dropped? I surely wouldn't.

This would no doubt give you an advantage over someone who does not know the bottom terrain, but that does not make it unfair! Some people spend hours, even days graphing unknown water. You can't consider their findings an unfair advantage just because they took the time to study what they can't see.

Sure, you know you put something there, but you also have to take the time to reidentify that spot when the water returns. No different then saving a GPS location.