Yeah, of course you'd lose $3 sunglasses in a river - it's because they weren't made to fit human heads. They were made with cost in mind. They fall right off. A good, fitting pair of Oakleys would hold on to you for dear life.
I've taken my most expensive (2720) Persols out Kayaking, and they were amazing. They took all kinds of abuse from salt water and paddle-hitting, and they came out scratch-free and clean with a little soap and water, and sunglasses cleaning solution for the lens. Plus, I can't imagine using anything other than polarized lenses on the water - there's just too many reflections on a sunny day.