I will post a few thoughts based on cruising in Maine for the last 22 years. I live on and boat out of Mount Desert Island.
There are many many destinations that are an easy day cruise from Rockland. However, one thing you will not find is lots of marinas or even docks to come along side. Most town harbors are full of moorings, some of which may be available to rent ($20-$40 per night). Once you head east from Rockland the only marinas you will find are in Stonington (Billings), Southwest Harbor (Dysarts) and Northeast Harbor (town owned). Expect to pay around $3-$3.50 a foot for a night at a marina plus a charge for electricity. Other than those places you will either be picking up a mooring or anchoring. You will need a dinghy.
As far as the season goes, you can start in May, but expect it to be cold on the water since water temps will still be in the upper 30s or low 40s. Also until about May 20th there won't be a lot of facilities open. Tourist oriented businesses on the Maine coast are very seasonal. Things generally get going just before Memorial day and start shutting down the week after labor day. The peak season is July and August, so expect crowds then in the towns. September is usually a great month for cruising since many of the tourists are gone and it becomes easy to find an empty cove to anchor in. October will have some nice days, but there will also be weather. Count on at least one real gale in October. By the end of October there will be few water based businesses open and very few boats in the water other than lobster boats. Almost no one boats in November because the weather can be a bit nasty (think 4-5 day long nor'easters with 40-60 knot winds).
Fog will also be an issue if you are not familiar with running in fog. However, you can always find places with little fog (in the lee of land), but figure on running in fog if you go out for more than a day. It looks like you have radar, so that is good.
Night boating will pretty much not be an option because of the lobster gear. Almost no one goes out at night unless they have a cage around their prop and even then you can get hung up if you have an unprotected rudder. Even lobster boats get hung up.
If you haven't cruised in Maine before you will be intimidated by the lobster gear. It is everywhere, particularly from July through September. In May and June the lobster gear is mostly in deeper water, but as the season progresses it moves into shallower water. If you don't have a cage around your prop you will snag some lobster gear during your first season until you learn how it works. Get someone to explain toggles to you and buy a LONG sturdy boat hook. However, once you figure out how the gear is rigged you will be fine. I do not have a cage around my prop and seldom snag anything.
As far as places to go, the best boating area in Maine is from Penobscot Bay to the Schoodic Peninsula just east of Mount Desert Island. East of Schoodic is nice, but there are not a lot of facilities and not all that many anchorages. On top of that the farther east you go the bigger the tides get and there are more places where current will be an issue. However, down east is spectacular and worth a trip, but be prepared to run in fog. The cruising guides describe down east as being "out there", but it really isn't.