marinas on northern chesapeake

I recently cruised the Chesapeake for the first time. We made a run from Havre
de Grace to the east end of the Sassafras, up the C&D Canal just past the 213
bridge, and back to Tidewater marina in Havre de Grace.
I became so enchanted with the bay that I have decided to slip our 25' Regal
pocket cruiser somewhere on the northern reaches of the Chesapeake. I would
prefer an area where the water runs fresh, as to keep the motor flushed upon
return to the marina. How far south does the fresh water turn to salt?
Tidewater is a fabulous marina facility, but at $3K annually for my 25 footer,
it's too expensive for my wallet. Can anyone recommend a nice marina, in the
northern reaches, with annual fees of about $80 per foot. Maybe my
expectations are unrealistic as to how much marina for the buck I'm going to
get. Thanks for any suggestions.

Mike Howard
"89" Regal 255 Ambassador