Hi Suddethb, congratulations on beginning to venture out into open water.
I don't have any specific advice for the D.C. area, but I do have some general suggestions:
1.) Your local triathlon club should be able to point you in the right direction. I am not a triathlete, but I joined a local club for just this reason. My local club organizes regular open water practice swims.
2.) Contact a Masters coach in your area. He/she should be able to point you in the right direction.
Good luck, and be sure to let us know how it goes.