Discuss Bonestructure's answer to: Why does one's blatter get so full when swimming?

It doesn't get more full, save by the body's normal processes. But it can feel more full. Part of this is because water always makes us feel like peeing. Part of it is that the water pressure squeezes down on your body and makes the bladder feel more full. It's not a lot of water pressure, close to the surface, but it's enough to do the job.