Your first problem is that
Maine has far fewer
marinas than farther south. And rental moorings don't typically come with launch
service. The only one I can think of offhand with a launch like a Crosby 26 is Handy
Boat in Falmouth Forside.
But if there is a marina
dock, it's likely to have a float as the 15ft tides make climbing a pier face at low tide difficult for anyone. But those floats also mean a long steep climb up a
gangway at low tide that might challenge your father too. The longer the
gangway the less steep. You could use
Google Earth to estimate the gangway length.
My favorite "base of operations" is Rockland. They have several marinas with floating docks. It's a nice town to walk around in that will be easy for your father. Rockland Public Landing has two gangways - the newer of which is very long and easy to negotiate. They need it as a small 100 passenger
cruise ship docks there one night a week. Some of these passengers are quite elderly. Northeast Harbor has a very nice marina too and access to the free National Park Services busses that would be easy for your father.
But you will miss a lot of what makes Maine a great cruising
destination if you limit yourself to marinas. I would consider mounting a handrail in your
dinghy. This allows you to come into a beach or float and step over the
dinghy tube while still holding the bar. It makes all the difference. If you're over 50, you might find yourself using it too

Something like this:
Ding-EZ Home