I found one once on the beach of Litle Saint Simons Island, Georgia. The note came from some class project in S. Carolina, and had been launched about 3 months earlier. I wrote back to the kid as instructed, but never heard back. It was a bit exciting to find a note in a (plastic) bottle... would have preferred to find an IBM stock certificate from 1946.
Lol. I tried the letter tied to a balloon method when I was younger. I never got a reply back.