Brad,
I fish the Potomac quite frequently, and YES I can say there are plenty of fish in there. Just last Thursday I was wet wading below dam 4, which is about 7-8miles above harpers ferry and caught a nice size rainbow inc a dozen or so 10-12 smallies. The potomac in my opinion fishes very well thru the summer and compared to the suskie I think the fish are healthier, no legions or things of that nature. I never have fished down by harpers ferry mainly b/c of all the people who float that section, i.e. tubbers, rafters, etc. I mainly work the williamsport area and it holds many nice bass, I even caught walleye, carp and catfish on the fly up there. Tight lines!