Yes, there are definitely rules about foster homes with swimming pools and hot tubs! Among other things, either the water has to be completely enclosed by a fence, separating it from all doors leading out of the house, or the doors leading out of the house to the pool area have to be alarmed in a kid-proof manner.
Because my back yard is rather small, a separate fence around the pool isn't practical. Instead I have an alarm on the sliding glass door that leads to the backyard. I also added a "pool eye" device that sits in the water when we're not using the pool. This device produces an ear-splitting alarm, both inside the house and outside the house, whenever it detects waves created by something over about 25 pounds hitting the water. I like the idea of having layers of protection. My county's social workers seemed to appreciate it, too.