Originally posted by Host Walt
Although I have no idea how it's done, many ships have saltwater pools on board.
Salt water maybe (although I have never come across it on any of the ships I've sailed on) but a swimming pool needs all sorts of hygeine treatment so they wouldn't just scoop water out of the sea and put it in the pool. Pools are regularly tested for disease and cleanliness so using just seawater wouldn't be an option.