It is my observation that very large individuals LOVE to get in the water. It is the only time that they are weightless... Fat people float really well and don't get cold too fast in cold water. When I used to guide lots of paddle raft trips the trick was to let the fat people swim in the river, but then to find a place where they could climb up on a rock and straddle the tube to get back in. Pulling a 500 pound person into a raft from deep water is difficult, and can injure their shoulders or your back.

Here's today's story about a very large man who went tubing:
http://www.twincities.com/allheadlines/ci_6394709?nclick_check=1