The Gulf of Maine ranks high in a list of the world's most productive seas, rich in a conglomeration of life uniformly bound together by their dependence on the cold, rich, and relatively unspoiled waters of this marine ecosystem. Shared by two federal governments, three states and two provinces, the Gulf of Maine illustrates the clash between political boundaries and those set by nature. Its fish, birds, marine mammals and other living organisms have never seen a map of the Gulf; they have no recognition of its man-made lines.

The waters of the Gulf are tightly linked to their watershed. A body of water, whether it is a puddle, lake, bay, or even a sea, has its own watershed. A watershed is the land area — similar to a bowl — in which water collects. It may run over the surface or sink into the groundwater, but much of the rain that falls and the snow that melts within the Gulf of Maine watershed will ultimately join the Gulf's salty waters.

The Gulf of Maine watershed is large (69,115 square miles or 165,185 square kms), encompassing land within Massachusetts, New Hampshire, Maine, Quebec, New Brunswick, and Nova Scotia. The entire state of Maine lies within the Gulf of Maine watershed, thus, much of the snow and rain that falls in Maine — and the polluted runoff from lawns and hard surfaces — in that state will eventually enter the Gulf.

Within this vast watershed lie many renowned rivers. To the west are the Charles and Merrimack rivers; to the north are the Saco, Androscoggin, Kennebec, Penobscot, St. Croix, and St. John rivers; in the east are the Kennetcook and Annapolis rivers. Of these, the serpentine St. John river system drains the largest area of land, followed by the Penobscot river system. The combined output of the Merrimack, Saco, Androscoggin, Kennebec, Penobscot, St. Croix, and St. John rivers pour 250 billion gallons (946 billion liters) of freshwater on average into the Gulf each year.

The lands in the Gulf of Maine watershed are as varied as any in the continental United States. In northern and western New Hampshire and Maine are some of the highest peaks in the eastern U.S. To the south, metropolitan areas crowd the coast from Massachusetts eastward though New Hampshire and southern Maine. The popular sandy beaches of these states draw millions of visitors each summer, leading to ever expanding development.

The population pressures lessen as one travels farther east, up the Maine coast. Here rocky headlands and granitic islands offer the drama and beauty many associate with the rugged Northeast. Farther east, in New Brunswick and Nova Scotia, the landscapes show the effect of the world's highest tides. In the Bay of Fundy, which separates Nova Scotia and New Brunswick, the tide may rise by as much as 54 feet (17 m) in height. By contrast, tides in Boston Harbor will vary by just 9.5 feet (3 m) from high to low tide. At low tide an array of sandbars, vast mud flats, and even ancient forests drowned by the rise of the sea are exposed.

The Gulf of Maine also features a number of important bays and estuaries. Foggy Passamaquoddy Bay, the northeastern most bay in the United States, is rich in shellfish and other benthic life. Penobscot Bay, in Maine, is the second largest embayment on the Atlantic coast of the United States. On its eastern shore it features Cadillac Mountain, part of Acadia National Park. Great Bay, in New Hampshire, is an estuarine nursery for many commercially valuable fish species. Its eelgrass beds provide habitat for juvenile scallops, shrimp, and other species. Massachusetts Bay is home to schools of humpback whales each summer, which come to dine on sand lance on Stellwagen Bank.

Several marine and estuarine preserves around the Gulf highlight the rich variety of life found there. Stellwagen Bank, a National Marine Sanctuary, is located offshore in Massachusetts Bay. The Great Bay (NH) and Wells (Maine) National Estuarine Research Reserves provide visitors a peek at the complexities of estuarine ecosystems. Nine National Wildlife Refuges (including Moosehorn, Petit Manan, Rachel Carson, Great Bay, and Parker River) offer undisturbed sanctuary to rare seabirds and other denizens of the Gulf. Along the Canadian coast, three National Wildlife Areas (Boot Island, Shepody Bay, and Shignecto) and Fundy National Park highlight the remarkable landscapes and biologic communities found in the region.