Hi Gburg,
I live in your same neck of the woods and have been considering purchase of a pontoon mainly because I want my bottom out of the water. I would use the tube or pontoon on the quieter waters of the upper Potomac (White's Ferry, Violet's Lock, Pennyfield Lock Sheppardstown, Petersburg , Harper's Ferry) and some sections of the Monacacy like Buckeystown area or even around Frederick. The Potomac has its origins in West Virginia and is a huge roaring river but it also has sections where there are wonderful areas of calm water. DO NOT be fooled by some of those sections which appear calm because some of them have very treacherous undercurrents that claim lives every year. I am NOT suggesting that you take that tube into any area without first getting very specific knowledge/information of where you are intending to float. The Potomac can be very deceptive and downright dangerous.

One suggestion I would make is that you invest in the books by Ken Penrod or even his video tapes by the same titles. He is a bass guide and fishes mostly hardware. He does touch on Flyfishing in some of his chapters though. His knowledge of the river is unmatched and his info is very specific.

Fishing the Upper Potomac / Penrod
Fishing the Tidal Potomac / Penrod
Tidewater Bass Fishing / Penrod

I believe that he may now have a fourth book out but I am not certain about that. There is also a map available that accompanies the Tidal Potomac book. It should be available from any good fly shop (Angler's Lie) in Arlington, Va.
You might also want to purchase an annual Fishing in Maryland (it is a magazine type manual which includes some maps and directions and is usually available at grocery and drug store magazine racks). The magazine has a wealth of info covering the entire state and all forms of fishing including saltwater fishing in the bay areas, rivers and community ponds. I have often thought how much fun it would be to float Needwood Lake in a toon and even the C & O Canal. This should be enough to get you started on an exploring and reading adventire that will take you through many seasons and maybe even the rest of your life. Have fun , be VERY careful and get in touch again.