In a Coast Guard drownproofing class I took in the '70s they told us about a guy who spent 2 days floating in the Mediterranean after he fell overboard from a Navy ship.

Coast Guard "downproofing" didn't involve any equipment. It was a survival technique that let you float and breath with minimum effort. To pass the course you had to go out to the middle of the swimming pool and stay there for an hour. I though it was going to be an ordeal. I promptly went to sleep and napped the whole hour.