I have never seen these before, and if they are going to be filled with water, then they must be basically an outside toy, or you would have water all over the house. I have no idea how you would keep them sterile, and still not use anything that is dangerous to the dog. Maybe peroxide, and then rinse it out good, since that is not harmful of a dog swallows a bit of it, and it will disinfect it.
I think frozen would not be good for the dogs teeth, so my guess would be just give them a bowl of ice water instead. My dogs are inside when it is hot anyway, so they would not use the toy full of water.
My dog stays inside with us for most of the day too, summer or winter. But I do know that there are owners who keep their dogs outside during the day while they're gone. I know that even if they have shade and shelter, things heat up pretty quickly for those little guys.
I had a quick look an Amazon and there is a real mix of reviews, but not great overall.. I think people mainly say their dogs destroy them too quickly and they are harder then expected. If your dog likes to destroy things then maybe not a good idea. I would also be worried about the mold too...
I think it's a lot more fun to have toys float in a kiddie pool than give them toys that have water and foam in it. And good point about the foam. It think it can be prone to molds.
A friend of mine puts balls in a kiddie pool when it's really hot outside and allows her pups to play in it. It encourages them to go to the water and at the same time keeps them cool. Of course, the water is only as high as their legs.