Machine Aquatics swims at Oak Marr RECenter, which is in Fairfax. Most of their swims are very early - 5am. I have swum with them on a few occasions. They have a pretty good mix and a good coach. It is more straight-swimming focused, however, than triathlon focused.
I swim with the Sunfish Masters at the Freedom Center in Manassas. It's a great group of people and the coach works wonders on your stroke if you want him to push you. Everyone has different goals, and he'll help you with whatever you're doing.