Can you be a human battery cell while in the swimming pool?

There are enough electrolytes in blood, saliva, urine, and many other bodily fluids that they can be used to make working batteries. However you need to take two dissimilar metals (e.g. copper, zinc) and immerse the two metals in such bodily fluids to make a battery. And such metals are not present in the situation cited in the question.

No, a human can not be a battery. A battery uses a chemical reaction between two chemical components to produce a voltage. The human body does not contain enough of these compounds to produce a voltage compared to a 1.5 volt dry cell. Being immersed in water will not effect the fact that the human body does not generate voltages of the magnitude of a dry cell battery.