Yes, all the pools on Century are salt water. The big Princess ships have fresh water pools. ie: Grand, Golden, Caribbean. There are probably others that have fresh water as well, but the majority are salt water. You don't need to take towells, they will give you all you need, and want. The tap water on board isn't the best, but they do offer bottled water (for a price), so you can take it yourself, if you don't mind carrying it. Century is a great ship, and no doubt you'll have a great time. We really like the dining room, as it's absolutely beautiful , and the food is outstanding. Entertainment excellent also.
You will receive a beach towel in your cabin for each passenger. That is for the entire cruise. It does get replaced when used. Do not take your beach towel to the pool. First of all, if the towel ends up missing, you are charged for it. Second, there are fresh towels by the pool.
We found the water good to drink. You will have a carafe of chilled water in your room as well as a bucket of ice. These are refilled by your steward.