Dreamt of swimming at a public access lake. I was a little off from the main beach, but within easy eye-shot of it. I threw one of the tennis balls I had with me in the direction of the other swimmers. Suddenly, a short, explosive sound rang out, much like a gunshot or a loud snare drum, startling everyone else. I threw another tennis ball and noticed that the sound rang out as the ball hit the surface of the water.