Dominica, Caribbean: Bubbling Volcano
Volcanic activity is alive and well on the Lesser Antilles island of Dominica, and nothing compares to snorkeling in it. Check out the island's nine active volcanoes and its boiling lake—the world's second largest—or head straight for Champagne Reef. On Dominica's southern tip, near Pointe Michel, this snorkel site is easily accessible from shore. You can hire a guide and rent gear right on Champagne Beach. Swim out over the seafloor's geothermal vents that release streams of warm, tiny bubbles into the 82-degree water. You can hear the hum and feel the fizz on your skin. It's like swimming in a warm glass of Champagne. Reach into one of the 110-degree rock fissures, or let your guide point out the frogfish, seahorses, and other all-star marine life in the clear bay.
Getting there: Several airlines offer connecting service from the U.S. to Melville Hall Airport on Dominica's northeast coast.
Bay Of Naples, Italy: Roman Ruins
Riding out into the Mediterranean Sea on a Zodiac, you'd never know an ancient city sits in the sand and silt just 400 yards off the coast of Pozzuoli. The boat stops at a nondescript orange buoy that marks the spot. Roll overboard and you'll see parts of Baiae's residential areas that were submerged when volcanic activity in the 16th century collapsed the coastline. Now an underwater archaeological park in 15 feet of water, Parco Archeologico Sommerso di Baiae includes villas with gorgeous marble mosaic floors. At the shallowest of the five sites, snorkel tour guides will dive down, brush away the sand, and retell stories about the statues, the columns, and the street lined with taverns that served this busy port in its heyday. The other four sites are great for scuba diving and freediving.
Getting there: Pozzuoli is six miles north of Naples. Meridianafly flies direct from New York's JFK Airport to Naples in the summer. Other U.S. airlines offer connecting flights year-round. Or, fly into Rome and take the Eurostar train to Naples.
Kona Coast, Hawaii's Big Island: Manta Rays
Every night after sunset, a dinner-theater show happens just offshore Hawaii's Kona Coast. Microscopic plankton rise toward the ocean surface to set the scene. Charter companies bring underwater lights to illuminate the main dish and attract more. Onlookers adjust their masks, snorkels, and cameras then wait for the action. From out of the darkness, almost on cue, as many as 12 manta rays, some with wingspans of more than 15 feet wide, swoop in to feast on one of their favorites. These toothless, barbless wonders float and swirl through the water, just inches away, with hypnotizing grace as their gaping mouths scoop up the plankton. Fair Wind's nighttime manta tours depart from Honokohau Harbor, just north of Kailua-Kona, and go to active feeding areas, usually in Garden Eel Cove or in front of the Sheraton Keauhou Bay Resort.
Getting there: From Kona International Airport, it's a 15-minute drive south to the Honokohau Marina and 30 minutes to the Sheraton Keauhou Bay Resort.
Truk Lagoon, Micronesia: WWII Ships & Airplanes
Two years and two months after the Japanese bombed Pearl Harbor, the U.S. attacked the Imperial Japanese Navy base in Micronesia's Truk Lagoon, sending 60 ships and 275 airplanes to the ocean floor. The ghost fleet, now coral-encrusted in clear waters, has long been a bucket-list destination among scuba divers. But even snorkelers can glimpse frozen-in-time history in this protected lagoon. Hover above one of the 15 ships and planes poking up near the surface. See tankers and gunboats with holds still filled with ammunition, gas masks, and human remains here in the biggest graveyard of ships anywhere.
Getting there: Continental Airlines (merging with United) runs regularly scheduled flights from Guam and Honolulu. Taxis, hotel airport shuttles, and rental cars are available in Weno, the capital city of Truk state.
Crystal River, Florida: Manatees
A sure sign winter has officially begun in Florida: the arrival of manatees. When the Gulf of Mexico drops to 68 degrees, they return to the warm springs in the Crystal and Homosassa rivers. It's only a four-degree difference, but for pudgy marine mammals with surprisingly little body fat, it's a significant shift. Here, in clear water less than seven feet deep, they're as excited as puppies to interact with others after what can be a solitary summer. They love attention and early mornings. Book the 6:30 a.m. snorkeling tour, when they will swim right up to you (in slo-mo), roll over, and even occasionally rub against you. Depending on the day, you could see 15 or only a couple. Best time to go: November through March, although some manatees stay in the river year-round.
Getting there: The city of Crystal River is a 75-minute drive north from Tampa and a 90-minute drive northwest from Orlando.