The Seabrook-Hampton Estuary is the largest expanse of salt marsh in the state. You will find numerous salt marsh birds here as well as many migratory species. The marsh can be easily accessed from NH Route 101 East and NH Route 286. US Route 1 and US Route 1A also run parallel to the estuary for several miles. There are no developed sites but there are plenty of areas where you can pull over with a pair of binoculars.
The marsh is abutted by many cottages many of which are available for rent.
Of course the beach is always nearby; Salisbury: