As far as swimming in the pools, it really doesn't matter whether you use fresh or saltwater. It only matters when you get out and begin to dry. If you don't rinse off really good, the salt will dry on your skin, turn white and dry your skin out and make you itch. Unless you're really short, you don't need to know how to swim. The deepest part of any pool is 5 feet.
It is more expensive to maintain fresh water pools since the water comes from the desalinization plant on board the ship. Most of the pools are 5 feet deep with the exception being Neptune's Reef pool which ranges from 5 to 7 feet on the Grand Class ships.