George,
To fill the swimming pool, they actually used the sea water from outside the ship. They constantly replaced it ... water flowed in from the outside and would flow out again. Since the water around the ports and busy shipping lanes was really dirty, they didn't fill the pool with water until they reached the open sea where the water was cleaner.
Daniel.
PS. The reason that I even mentioned the pool was that the 1st class cross channel passengers could use and explore all the 1st class public rooms. They could go and visit the pool, but it would have been empty and they would not have been able to use it.