When a person drowns they go straight to the bottom, it's not for about 24 hours before they come back up to the surface
24 hours? Damn, that's a long time. I didn't realize it took that long.
Wiki Answers. Not sure how accurate, but sounds good.
After death, the organisms/bacteria within the body will metabolise and multiply, producing lots of gas in the process. Most of this gas is unable to escape, so the body expand and float from all of the gasses present. The body will then sink again once it has decayed enough to allow the gasses to escape.
ETA: More Wikianswers...
Dead bodies float on water much the same way living bodies do. And yes, living bodies do float on water. But if you panic and exhale, you will sink. IF you stay calm and keep your lungs inflated you will float. This is because of density. Anything less dense than water floats, and anything more dense than water sinks. Our human bodies have a density close to that of water and we WILL float if we keep our chest expanded and lungs filled with air. When people drown, their lungs are full of water and they sink. After a number of hours, up to a few days, gases from decomposition trapped inside their body make them buoyant and they will float. Then carrion eaters, blow flies, and other flesh eating creatures will puncture through their tissues, releasing the trapped gases, and causing them to sink again.
This post was edited on 10/9 at 1:43 pm