Portland Airport (http://portlandjetport.org/), which is actually in South Portland, Maine, is the
closest airport and has most major airlines, including Southwest. The airport code is PWM.
The next best, and sometimes better, option is flying into the airport in Boston, Massachusetts (BOS), which is a little less than a 2 hour drive from Portland. Concord Coachlines bus runs hourly routes to Portland (http://www.concordcoachlines.com/). Driving your own car from there is not difficult, except during rush hour times.
The closest hotels to Yarmouth Research are in Portland, South Portland, or Freeport, Maine. They are all about a 25 minute drive (easy routes, little traffic). The advantages of each are:
South Portland - Hotels in this area are convenient to the airport and the shopping mall area, which is surrounded by many national chain restaurants. A good choice for a quick stay with late arrivals or early departures from the airport.
Freeport - Freeport is about 30 minutes north of the airport and 20 minutes north of Yarmouth Research. Freeport is famous for being the home of LL Beans and about 100 other outlet stores. It is a nice little town, easy to walk around. The hotels shown on the northern part of this link would be near the shopping mall area, and those more south on Route 1 are just a few minutes closer to the lab but would not be walkable to the stores. Freeport is a good choice if you are bringing a significant other who likes to shop.
Downtown Portland - This would be my recommended choice. The waterfront area of Portland has many world-class restaurants as well as some local hangouts. Restaurants that get lobster right off the boats, a ferry service to take a sunset cruse around the harbor (http://www.cascobaylines.com/), and some good shopping are available. It is convenient to walk everywhere from the hotels shown below and is safer than most cities. Hotel prices can unfortunately be high during tourist seasons.