In Georgetown right near the harbor there is in fact a canoe/kayak rental concession called the Thompson Boat Center (they have a website, google it). You can basically canoe/kayak a mile or two north, but you don't want to go as far as the little falls as it becomes dangerous there. Ask Thompson Boat Center for more advice. I also would not canoe or kayak much south of D.C. on the Potomac as the river becomes wide, the sun will shine hot, the river is tidal, and the water can be choppy. Some of the best local places to canoe or kayak are the Occoquan Reservoir (Fountainhead Regional Park), the Potomac River NORTH of great falls (Riverbend Park and Algonkian Park), lakes like Burke Lake, and probably the Shenandoah River.