Why do red blood cells swell and burst when placed in a hypotonic solution as water, and why don't humans burst when swimming in water?
Water will cross a cell membrane, and go from an area of less solute concentration to an area of more solutes, so that the concentrations are equal. This is why a blood cell will burst when placed in pure water--the water goes into the cell, where there are more particles. In a living system, though, energy is used by the organism to keep this from happening. Active transport keeps the fluid level in the cell to the level that allows the cell to function. This is one of the key differences in living systems as opposed to non-living; living systems utilize energy to keep themselves in a functioning state. When an organism dies, it is no longer using energy to keep itself functioning, and starts to decay.
When swimming, our blood system is protected from water by our skin. It is a barrier to the penetration of water and so water and our blood do not come into contact.