I'm on the MoBo cruise! I booked onboard the 40th anniversary cruise. I thought, "I can drive to Montreal or Boston easily, but I want to take this trip with the Olivia crowd!" (the 40th anniversary cruise was my first Olivia trip)
I was born and grew up in Maine, and I still have family in the Portland area. If you plan to drive up the coast, you'll head north from Boston on I-95, continue briefly through New Hampshire, then pick up US Route 1 in Kittery, Maine. Ogunquit is a short distance up the coast from Kittery. From there, you can expect about a 30 or 45 minute trip to Portland using the coastal route (Route 1). It's really scenic, and you'll find a lot of interesting spots to check out on the way. PLENTY of lobster! In May, the traffic should not be bad.
Portland itself is a very interesting city, but you'll also get an excellent "flavor" of the Maine Coast when the cruise ship docks in Bar Harbor. I overhead some "Maine-ahs" on the 40th anniversary cruise talking about planning a big welcome in Bar Harbor for the Olivia Ship! But if you have the time to spend a day or so poking around Portland, you won't regret it. There is a substantial lesbian/gay community in Portland, though I don't believe Portland has quite the same cachet as Ogunquit.