Gerry E. Studds Stellwagen Bank National Marine Sanctuary, stellwagen.noaa.gov, (781) 545-8026
Deposited in the last Glacial Age, the bank is an undersea sand, rock, and gravel plateau just north of Cape Cod, Massachusetts. Most visitors here are whale watchers (though dive operators and charter fishing boats offer trips to the bank, too) hoping for humpback whales (most common in summer), and from January through April and October and November, a glimpse of the endangered northern right whale. Plenty of other wildlife exists here, too, from 40 species of seabirds to pods of white-sided dolphin to the pancakelike mola mola (ocean sunfish).

Thunder Bay National Marine Sanctuary, thunderbay.noaa.gov, (989) 356-8805
Some 160 shipwrecks lie in the northwest corner of Lake Huron, not far from the town of Alpena, Michigan. In water as shallow as 12 feet and as deep as 180 feet sit 19th-century wooden schooners, steel-hulled steamers, and modern-day freighters. Snorkelers ogle the shallow wrecks and divers descend to the deeper ones, eternal silence wrapped in still water. If you prefer to stay dry, there are glass-­bottom-boat and lighthouse boat tours to try.

Hidden Opportunity
America's 13 national marine sanctuaries are watery wonderlands welcoming visitors, for the most part, with open arms (in some sanctuaries, certain areas are off-limits). The office of National Marine Sanctuaries' website is the best place to start (sanctuaries.nos.noaa.gov). you can also call the sanctuaries to find information on commercial outfitters. In addition to the Channel Islands, Florida Keys, and monitor national marine sanctuaries, these 10 offer a plethora of outdoor fun.

Cordell Bank National Marine Sanctuary, cordellbank.noaa.gov, (415) 663-0314
Upwelling makes this offshore granitic bank (43 miles northwest of San Francisco) a feeding ground for sea life. From June to November, humpback and blue whales feed in sanctuary waters. The waters are a year-round birder's paradise, with seabirds from around the world, including the rare wilson's storm petrel and streaked shearwater. Sportfishing for albacore and salmon is popular; most sportfishing and whale-watching boats leave from Bodega Harbor.

Fagatele Bay National Marine Sanctuary, fagatelebay.noaa.gov, (684) 633-7354
Fagatele Bay on Tutuila Island in American Samoa is the smallest (one-quarter square mile) and most remote sanctuary, so visiting takes some effort. But from June to September, the pristine waters are home to southern humpback whales arriving to court and calf. Dolphins, hawksbills, and green turtles also frequent the waters. Boats leave from Pago Pago harbor for one- to two-hour rides to the bay.

Flower Garden Banks National Marine Sanctuary, flowergarden.noaa.gov, (979) 846-5942
Located some 110 miles off the coast of Texas and Louisiana, Flower Garden Banks may be the oddest of the sanctuaries - three salt domes hump up from the floor of the Gulf of Mexico, toupeed by the northernmost coral reef in the continental United States. The banks are a diver's paradise, and the seven- to eight-hour trip to the area (most outfitters leave from Freeport, Texas) is worth every minute. From January through April, scalloped hammerheads visit, and in August or September (depending on the date of the full moon), the area erupts in a lovely undersea snowstorm, the result of a mass coral spawn. Outfitters - who know the approximate timing - offer spawning cruises.