A colleague in the office recently introduced me to a Swiss invention called the Wickelfisch (or baby fish). Below is a video showing you how they work. They're so neat. So Swiss. And I obviously just ordered one. They're available via this US company for $12-20, depending on the size.
https://www.youtube.com/watch?v=K0QagvuIFSo
The video is of people swimming in the Rhein. Because swimming downstream in rivers -- and ending up in a different location -- is so popular in Switzerland, the Whickelfisch was invented as a way to keep your clothes and belongings dry. It also acts as a flotation device.
I'm not planning on floating down the Don River anytime soon, but a Wickelfisch looks like the perfect bag to bring to the beach. Especially if you don't want to leave your stuff unattended to while you go for a swim.
Of course, the other neat thing about this video is that it shows you how awesome it is to have clean and swimmable water running through the middle of a city. Most people I know are afraid to swim in Lake Ontario out of fear they might grow a 6th toe.
That's too bad.