Do a salt water pool. A little harder on the equipment, but better overall footprint.Actually, it's not so bad. The ship uses a new dock that doesn't require dredging and is a fair bit away from the island itself. The trash is returned to the ship.
Besides, it's not about the overall impact. It's about not making that impact even worse. Disney had to do a lot to convince the Bahamian people that they were going to be as environmentally friendly as possible and that they would develop a small percentage of the land. Running a pool requires a lot of energy, a lot of chemicals and a lot of space. Who needs it when the ocean is right there? Better to spend your limited power and land budgets on other features.
Some people like pools and not sand and a pool doesn’t take that much space In the grand scheme of things.