I am the retired harbor master from Castine Maine. We are too far from Boston (4 hours) for anyone to regularly commute. Boothbay would be OK if you like the sailing in that area and Muscongus Bay, but Rockland has better access to the glory of Penobscot Bay, its islands, and the inside passages to Acadia--IMHO these are the preferred cruising grounds of Maine. Calculate the drive and make a decision. You cannot legally buy a mooring in a town mooring field in Maine (State law) from a person to person transaction, so be careful with anyone who sells you their mooring--they could be selling you just the gear, not the position in the mooring field. In a town mooring field you rent the space for the mooring from the town (generally $100 to $300 a year), and buy and own your own mooring gear as specified by the harbor master. The harbor master has local and state laws that govern who gets to moor where and when. Even some lifelong Mainers fail to understand that they may own the mooring gear but they do not own the place they set the mooring in a town harbor (all the land under the water belongs to the State) and they cannot transfer it without the permission of the harbor master. It might be possible to buy a mooring in front of someone's house along the shore where there is no town authority monitoring moorings but this is rare. Harbor masters in Maine are actually law enforcement officers though most do not carry weapons or have uniforms. Some are very sloppy and others adhere to the letter of the law.
BTW: Sometimes renting is cheaper than owning. A complete set up of pendant, mooring ball, top chain, bottom chain, rock or other grounding mechanism is $2 to $4K to purchase and install--and harbor masters often require that you them serviced (which generally means replacing at least the top chain) at least every 3 years. Buying someone's used equipment can be a crap shoot. Generally, you want to buy the mooring gear from the folks who place it and service it.