The problem with a gallon of water is the DILUTION of the blood cells resulting in dizziness and faintness, since there is a decrease in the amount of oxygen being circulated in the thinned blood.

Certainly a half of gallon of fluids would be acceptable, but not all at once. Sipping Some before and after the sessions would be indicated, since many people do not get enough fluids in general.

Look up hyperhydration.