'Salt water' pools are still using chlorine, just via a continuous process that maintains the right level of chlorine (splitting the salt: NaCl). Much easier to maintain. If the system has a super chlorination function (periodically gives a burst of increased chlorine, e.g. overnight) you don't have to shock the pool very often.

Big municipal pools have continous chemistry monitoring so they can achieve the same effect with less electricity. They can also increase the flow rate when there are more people etc.