Same thing with my 2 Weimaraners. Once the pool is opened, and the water is clean, it becomes their never ending water bowl. Even though we put down fresh water throughout the day, when they are thirsty they'll whine to go outside and drink from the pool. They've been doing this since they were pups, and they are now 10 and 13. Interestingly enough, while I'm cleaning up the algae that grew over the winter they won't drink the water, nor will they swim in it. After about a week though, the pool is theirs.
I too was concerned about the long term health ramifications of this routine, but my water is usually very controlled. I keep my free chlorine at 2-3 ppm, my cya is never really that high either. I joke with my wife that it's probably safer to drink the pool water than our unfiltered tap water.
I tried to train them to not use the pool as a drinking water source, but they can be pretty stubborn dogs. Good luck.
Tom