Not that you should ever be out swimming when severe thunderstorm warnings and tornado watches are in effect, but if you ever had an inkling, maybe this video will scare you straight.
Take a look at this video of a massive hail storm that struck Oklahoma on May 16, 2010. It starts slow, but watch as the storm intensifies to where the swimming pool transforms into an angry cauldron under a deluge of hail stones.
I can't imagine what any cars that were unfortunate to be parked outside looked like in the aftermath of that storm. (Oh wait - there are some videos below)
There were several reports of baseball-to-softball-sized hail across the Oklahoma City area with "hail drifts" several feet deep in some areas. In addition, wind gusted to 50-60 mph. (Read more on the storm on this National Weather Service site
Here are some other videos on YouTube also from that same storm. (Note: these are unedited and some may contain off-color remarks from the video photographers in the excitement of the storm.)