I was diving a Florida spring earlier this summer and noticed the light mellow out a bit. Several minutes later, I noticed some flashing, and I looked around to see where the pair of cave-diving photographers I'd seen earlier had gone. Then I remembered. They'd left about half an hour ago... and I hadn't seen anyone else with photog kit...
I looked up from what I'd been studying to see the surface roiling from a torrential downpour and thunderstorm. The next thought was "Oh, *NUTS*, I left my sunroof open! Gotta get it closed." The thought after that was "Phooey! I haven't even done my deep stop yet." Accepting that the drenching of the inside of my car was something I could not change, I made my ascent with extra-long stops, and by the time I was running low on air, the storm had passed.
I lost four rechargeable AAs, a power supply, most of a cell phone (it's my emergency backup now), and a bunch of soggy stuff due to the storm. Nobody saw it coming, though, so I didn't have to kick myself. (Even the guys topside were taken by surprise.)
A few weeks ago, I was diving Morrison Spring, also in Florida, and I saw the signs of an impending isolated thunderstorm. I quickly got in the pool right as the rain was arriving, and there was only a tiny bit of drizzle when I surfaced (the thunder long gone). When I was coming back up out of the spring, I was treated to one of the coolest visuals I'd seen in a while. The sediment running down from the big sandlot parking/beach/etc. area was pouring into the spring run, and it created what I can best describe as an upside-down underwater haboob. The spring basin was still almost perfectly clear (the classes had bailed, of course) with this "dust storm" perfectly intact as it headed down the run.