There's a new OW swim in our area, in Columbia, MD: Choptank River Swim, distances of 1.6 and 3.6 miles. More info here: http://www.tricolumbia.org/events/?eid=32
I maintain a list of OW swims in our area here: http://marathonswimmers.org/resources/swims/northern-virginia-washington-dc-open-water/
If you sign up, make sure you beat the price increase. Sometime in late April, it goes up from $60 to $85.