Since monitors don't shed their corneas (like snakes do) the cornea is wet, and needs to last for the life of the animal. That being said, I have let my savannahs enjoy a pool on rare occasions, but I check first to make sure the chlorine level isn't strong. Public fountains are generally heavily chlorinated, and I would avoid these. If the animal is swimming out of fear, or if he is simply dumped in the pool, he may stay in the water even if he is uncomfortable. If possible, I would let the lizard enter the water of his own accord, and make sure he can climb out easily when he wants to.
I also understand that most monitors enjoy a frosty pina colada when sitting poolside JJ
PS -he will PROBABLY defecate in the pool while swimming.