Greater Boston has MWRA water but some local sources too. Cambridge, for example, has its own water supply, so does Winchester and I think Lynn too. MWRA water is probably similar to NYC water: both systems have well kept upcountry reservoirs where the water is clean enough to avoid expensive filtration systems. I've heard only four big municipal water supply systems in the US have been able to meet federal drinking water standards without filtration systems and two of them are MWRA and New York City. When Massachusetts created the Quabbin it acquired plenty of land around the reservoir, now forested watershed land that keeps all sorts of typical contaminants like lawn fertilizer runoff from entering the water. NY's reservoirs don't have as much watershed land and the DEP has had conflicts with farms and towns in the Catskills (where the reservoirs are) over runoff, also sewage treatment systems that discharge effluent into streams that feed the reservoirs. So NY water may be a little less pure but still good enough. Philadelphia, by contrast, takes its water out of the Delaware river, downstream of Trenton, Easton, etc. Yuck.