Anybody entering a swimming pool contaminates the water either from secretion or just from washing off.

Typical contaminated are
Sweat, skin flaks, mucous, hair ointments and urine. 

In addition there are contaminants and organisms from the environment of the swimming pools such as dust algae and microorganisms therefore the pool water has to be constantly filtered and disinfected. On average the human body will absorb half a liter of pool water every hour. So you can only imagine what sort of harmful contaminants are being absorbed into your body whilst swimming