I used to live on the river in Mt.vernon.Try the fort hunt area,also the anacostia river north,look at the area on google earth,lots of interesting stuff in the water there,looks like gun implacements and old buildings now underwater.Also across from mt vernon at fort foote park.You also have fort washington on that side too.Also on the maryland side right scross from yacht haven,there used to be a large amusement park with a very large ferris wheel.They tore it all down before i left that area in 1981.Im sure theres some goodies buried there.Also some small cove along the potomac south of ft belvoir the navy sunk a bunch of WW2 PT boats complete with engines,a friend raised up 2 of them and restored them to sell.If you find them let me know an i will come up and raise a few with you.The river has cleaned itself up quite a bit since i was there.It was really bad,so many dead fish that you could almost walk across the water on them.And i used to windsurf there in high winds and jump the 10 foot wake from the mt vernon tour boats.I wore a full wetsuit and had all my shots up to date.
Jason,
Check out the photo,the cords are on it,lots of shipwrecks to check out,let me know if you find anything there.