Try the masters group at Asphalt Green, 91st an York on the Upper East Side. I swam with them when I worked for Goldman Sachs in NY.
www.asphaltgreen.org/homepage.aspx
Click on SWIM
Then chose Adult programs, masters
Not sure where you'd be coming from, but if Manhattan, I took the 4, 5, or 6 (Green) and got off at the Lexington (86th street), then walked up 5 blocks and over from 3rd, 2nd, 1st to York.
When I was visiting NYC, I swam with the Hydras: http://www.nychydras.org/
Just contact them in advance of going. They were a very nice group with very cool coaches.
Bear in mind two pool rules somewhat unique to Baruch College where they swim: no shoes on the pool deck (but I think you must go through the pool deck to get to the lockers) and you MUST wear a cap. The latter is not an issue for women but I'm a guy and I don't normally train with a cap on. I had to borrow one...