After reading your question, I thought of the number of relocation clients (coming from overseas or out of the NE area) that I have worked with. The answer comes down to what type of town you are looking for based on what it offers and also what your price range is. Anywhere in the general NH seacoast area is going to be close to the ocean and also relatively close to most major communitng byways. You need to think about what type of property you would like and what you want directly around you.
Price will impact this decision greatly. Directly on the seacoast, the difference between a 4 bedroom 2000 square foot home in Seabrook will be significantly less than the same in Hampton Falls or Rye. If you would like a condo - you would be hard pressed to find any in Hampton Falls, North Hampton or Rye. Hampton, Exeter, Stratham and Portsmouth have dozens to choose from.
There is such a different feel in each town as well. Seabrook and Hampton have quite a bit more commercial development on Rt 1 as does Exeter/Stratham on Portsmouth Ave. If being close (within a 7 minute drive) to supermarkets, drugstores etc is imp., than these work well. Depending on where a home is in Rye, Hampton Falls, the outskirts of Exeter etc, it may be a bit more of a trek to get that gallon of milk.
Taxes vary greatly, the way the local government is run varies town to town and the population does as well. It is one of the great things about the area. Each town truly has its own personality.
I think that once you answer some of these questions, you will be able to zoom in on which towns will work best. It is a great place to live!!!
Better Homes & Garden Real Estate
No doubt your question, "Looking for our ideal Southern NH town: close to ocean (within 30 minutes); reasonably convenient for Boston commuting (train/bus/car); reasonable(!) property taxes; schools will only matter in 5 years." will bring a flow of Chamber of Commerce replies from the Realtors among us.
Although Seabrook is an answer to the question you ask, many of my clients seem not to want a Nuke Plant in their backyard. (Although that keeps the Seabrook taxes low). I personally like Exeter for it's cosmopolitan style, there is a train stop (and it's one stop to Boston) sense of history, good schools, restaurants and shopping. It's one town back from the beach communities, so the summer traffic is minimal, but as convenient to Boston and it's closer to Manchester airport.
Obviously it's you decision, but those of us who live here think the Exeter area is tops.