There are actually a number of lighthouses in Boston Harbor. Historically, there had been many more. Some are described at the National Park Service website: https://www.nps.gov/boha/learn/historyculture/lighthouses.ht...

Not all of these are "lighthouses", per se. Nixes Mate is actually just a concrete-and-paint reflector: https://en.wikipedia.org/wiki/Nixes_Mate