Diving in the Potomac is mostly about timing relative to recent rain events. Most areas you can find 10’+ of visibility. About the pollutants I’d suggest not diving immediately near the Anacostia River.
If diving near the Old Railroad Bridge piers you may have really good visibility and tides/currents shouldn’t influence you much. It’s less than 30’. There’s a deep hole (60’+) under the Tydings bridge on the east side of the island. Good vis. if not after a rain event.