Why does he need his guide dog in the water? The dogs can't guide you while you swim.

Why in gods name would he want his guide dog in the pool? The dog is not providing a service and dogs love water; trying to get a dog out of water is hard enough without trying to get it to do some work when it's in there. Plus, dogs have a tendency to drop hairs, and hairs have a tendency to clog up filtration systems.

Guide dogs, like all service dogs, must be unobtrusive. While a dog could be on the pool deck and be unobtrusive, I don’t believe a dog would be able to be unobtrusive to the other swimmers and staff responsible for cleaning while in the water. Therefore, I believe property management has the right to refuse to allow the dog in the water unless they allow other dogs in the water. Guide dogs are not trained to guide will swimming or protect from things like drowning. Many of my friends who swim have their guide dog stay on the pool deck while they swim, but that only works if the dog isn’t barking, crying, or jumping into the water. Because I can not control my puppy in trainings behavior while I am in on the water, my dog stays at home when I swim.

What service can the dog provide while in the pool?

This is taking it too far. They don't have the right to have the dog in there bothering other people because it is not trained to be in the pool. Its hairs will probably also shed some in the pool, getting into the filters. If it's not enough for the dog to remain near the pool, but out of the water, he shouldn't be there without someone.