Underwater training for astronauts is really important. It's the way in which they learn how to counteract weightlessness in space. i have experienced a similar training at an amateur level once, when i had been a member of Space Camp India. When i registered myself at http://spacecampindia.com/index.htm I knew that I would get an overview of astronaut training methods, but i never expected it would be so much in-depth and exhaustive. We were actually given oxygen cylinders and scuba gears. Later we had to step down the pool at the US Space and Rocket Centre at Huntsville, where we had to perform different activities - all underwater. Because of the buoyancy we had no control on our body movements and a small task seemed so difficult to perform. However, with lot of effort we managed to complete our task. Later we were informed that astronauts also face a similar situation in space. Due to weightlessness, one task which seems so simple on earth, would be really difficult to accomplish on space. Hence this is how they are trained to counter the situation.