I must humbly disagree. We have a "freshwater" pool in our backyard. When I'm on a cruise, I am so happy to NOT clean the pool that I don't care if it is freshwater or saltwater.
I cannot imagine the resources that ships would need to use to cleanse the water in a pool. My home pool is about 14,000 gallons. How much time and energy would it take to filter all of that water? Especially since most people just dip in the pool - you don't see a lot of lap swimmers. I'd rather see the cruise lines use that expense to improve the food!