A few years back, we drained the pools nearly every day, cleaned them, and refilled them immediately.
Those were in the good old days when passengers followed the rules and instructions about showering before entering the pool.
Then we had our fuel price crisis, and had to come up with ways to reduce fuel consumption. It costs us about US$400 in fuel to produce enough fresh water to fill the average ship's swimming pool. As soon as that info was known to the bean counters, we had a new schedule for changing the water in the pools. Now we can wait for weeks to get the pool water changed. We keep dumping more and more bromine and chlorine into the pools for safety, only draining the water when you can no longer see the bottom.
At the same time, North Americans have stopped showering before swimming. All that deodorant, makeup, body lotions, perfumes, suntan lotion, fecal matter, urine, and god only knows what else, end up in that pool for many days.
I will not even get close to one of those bacteria soup basins.