MIT is easy enough to get a membership if you know someone who went there (they just have to sign a form). The pools are the nicest around (better than Harvard’s). War Memorial is the public swim option for Cambridge residents but it has weird hours - so it either works for you or it doesn’t. The Central Square Y has a pool in the basement; I think it’s three lanes. It really isn’t a bad pool but it can be tough when it’s crowded