Yup, a lot of fun. I guess. I've never done it. But I've seen, and heard, plenty of people who have. On the water, that inner tube doesn't look so big, but when it boings out of the back of a pickup truck that's in front of you while you're driving your car at 55 mph, and gleefully bounces into your lane, it can look mighty huge, and lethal.
I know this to be true because it happened to me last week. Not to worry; I'm fine, my car's fine and so is the motorcyclist who was behind me. All vehicles and drivers are all Just Fine.
Like a surfboard breaking free from the roof-rack of a moving vehicle.
Well, that didn't happen. Not until the board shot off the roof and exhibited the equivalent of a gymnastic back handspring routine for several seconds before coming to rest halfway in the lane, halfway in the grass. I was so far back at this point, like a city block far back, that I had no worries about my car and that board colliding. As I passed the grounded board, the driver who was scrambling to get it off the road, waved and shouted, "I'm Sorry!" A tall, lean, tanned 20-something man wearing low-riding board shorts made it easy for me to forgive him. My kids waved back and talked excitedly about the sailing surfboard, my son exclaiming that when he got bigger he was going to trade in his boogie board for one of those.
I reminded him that he had to have a car first. With a dependable rack made specifically for surfboards.