If you want a ship to be stable in rough seas, it must have a very low center of gravity.
A low center of gravity requires most of the ships weight to be located as low as possible in the ship.
That's why the lower decks are built with steel and the upper decks are built with aluminum.
But swimming pools are a problem. Water weighs about one ton per cubic meter. Even a small swimming pool contains 80 to 120 cubic meters of water. A larger ship can have 4 or 5 pools like this on the upper decks. 400 to 500 tons of water moving on top of the ship in heavy seas can be quite unstable and dangerous. Larger pools would just make it worse - and even more dangerous.
Fresh water costs money. Whether you bunker it ashore or produce it onboard, you have to pay for it; anywhere between $2 and $4 per ton. Doesn't seem like very much - until you multiply that by 400 or 500, and then do it again every few days when the pools have to be dumped and re-filled. Larger pools just cost more money to fill and re-fill.
Salt water in pools doesn't cost much, but the chemicals required to treat that water cost a lot. Larger pools require more chemicals and that means higher costs.
Next time you cruise, take a few minutes to count how many fellow cruisers take a shower before they enter the pool. It should be easy. The number will be near zero. The amount of fecal material and urine that goes into a ship's swimming pool is shocking. The amount of chemistry that we are forced to put into that water to kill all the bugs is even more shocking. These pools are often just open sewers.
Larger pools only encourage more people to go in - and stay longer. This is not a healthy option.
All of that polluted and chemically treated pool water is dumped into the ocean every few days. Not exactly an eco-friendly procedure. Enlarging the pools just increases the amount of pollution we dump into the seas.
