This Sunday is the second annual Washington's Crossing: Swim Across the Potomac. I'm told spots are still open (price increase 6/1), but I'd move fast. This is a really fun race that starts on the Alexandria side of the Potomac near the Wilson Bridge and ends up at National Harbor (1.3 miles).
Very newbie friendly. If you are NTP, this would be a great opportunity to get some OWS experience in a race environment.