For dogs and cats that get 100% of their drinking water from a pool with 50 ppm of borates the total long term dose of borates will still be below the threshold of detectable symptoms.
Note that for humans typical health regulations require that you be way way below the threshold of detectable symptoms, which is not the case for pets in this situation. Humans are dramatically less sensitive to borates compared to pets, so none of this is an issue at all for people. Still, if you want to keep you pets as safe as you keep your children, it is best to teach them not to drink pool water (and that is for several reasons not just borates).