If you plan on diving more than once or twice, it's worthwhile to bring your own gear (if you have it); even casual snorkelers should consider bringing their own mask and snorkel for both fit and hygiene purposes.
Here are some of our top choices of places to explore life underwater.
Key Largo, Fla.
You needn't wait to leave the U.S. for your first opportunity to check out tropical waters. Take a day before embarkation in Miami, and head south to Key Largo. The island is home to John Pennekamp Coral Reef State Park, where you'll find the Christ of the Abyss statue. The 8.5-foot-tall bronze statue sits in about 25 feet of water, making it suitable for snorkelers, as well as divers.
Adjacent to the park is Molasses Reef, home of soft coral, elkhorn coral and plentiful aquatic life, including parrotfish, lobsters, tangs and jacks. The reef begins in only 15 feet of water, so snorkelers can easily get a peek. If wrecks are your thing, the purpose-sunk Spiegel Grove is one of the largest artificial reefs at 510 feet long. Note that it sits in about 130 feet of water, so it's suitable for advanced divers only.
Who It's Good For: Reefs and underwater statuary are good for snorkelers and all levels of divers; wrecks are suitable for advanced divers.
There are more than 700 islands in the Bahamas, and the island chain is a veritable buffet for scuba divers. You'll most likely be confined to Nassau or Grand Bahama near Freeport for day excursions, but never fear: there's plenty of underwater action to be had.
On Nassau, adventuresome divers have the opportunity to do a two-tank shark dive. On the first, divers descend onto a wall where the sharks will follow them to a feeding site. (The intelligent animals know what's coming.) On the second, the group drops as a whole to the sea floor and sits in a semi-circle while a trainer feeds the sharks from a bait box placed in the center of the circle.
At Grand Bahama, divers can experience tiger sharks up close on a two-tank excursion or stick with a gentler experience on the reef. Snorkeling's not great around Nassau due to development, but there's some coral to be seen at Love Beach.
Who It's Good For: Adrenaline-seeking divers who're itching to get up close and personal with the ocean's apex predator will enjoy this location.
Grand Turk, Turks & Caicos
Tiny Grand Turk is a gem for divers; do yourself a favor, and hightail it from the cruise port to Cockburn Town, where you'll find a number of dive shops strung along a sleepy oceanfront street. Just a few hundred yards from shore, the crystal blue water gives way to inky cobalt where the wall drops away to 7,000 feet. Most dives are on the west side of the island, with the reef starting in just 30 feet of water. Visibility averages 100 feet at sites like The Aquarium, where the reef rises and falls to create several sand canyons. There's healthy brain coral, boulder star coral and sponges, and divers will frequently encounter Nassau grouper, squirrelfish, green turtles and a host of small reef fish.
Visiting snorkelers will want to join an excursion to Columbus Landfall National Park, a protected marine sanctuary, or snorkel off Pillory Beach at Bohio Dive Resort. You'll likely see nurse sharks, conch and small tropicals.
Who It's Good For: Everyone, from novice snorkelers to experienced divers, will appreciate Grand Turk's wall.
The southeast region of the Dominican Republic, home to both La Romana and Samana, has experienced an explosion in vacation options, and scuba options have followed suit. The formerly sleepy fishing village of Bayahibe, about 45 minutes from Punta Cana, is the launching pad for many of the island's best dives.
At Penon, you'll find a reef ledge overrun with life, from green morays and nurse sharks to giant crabs. The wreck of the St. George sits in about 100 feet of water at the stern and offers penetration in the wheelhouse and hallways. Snorkelers will want to check out the coral around Saona Island, part of the protected Parque Nacional del Este.
Who It's Good For: Whether you're a novice snorkeler or experienced diver, you'll find something of interest.
British Virgin Islands
The B.V.I.s are not only a playground for sailboats, but for divers and snorkelers. The granite boulders of The Baths on Virgin Gorda are a must-snorkel, even for the hardest of hardcore divers. After that, take your pick from wrecks like the famous RMS Rhone, a royal mail steamer that went down with all 125 passengers in 1867 (the B.V.I.s' signature dive) or Spyglass Wall, which features lots of sea fans and small fish, such as damselfish, wrasse and fairy basslets. Keep a lookout for southern stingrays and passing tarpon.
The Chikuzen, a 246-foot-long refrigeration ship lying between Virgin Gorda and Anegada is also worth a few tanks; it attracts sharks, cubera snappers and a resident 600-pound goliath grouper.
Who It's Good For: This location is great for everyone from unseasoned snorkelers to experienced divers.
St. Thomas, U.S. Virgin Islands
It's one of the Caribbean's most popular cruise ship ports, but, luckily for divers, it's also a good jumping-off point for some great underwater viewing.
The waters around St. Thomas are littered with wrecks; the most requested is WIT Shoal II. Originally a warship, it sank in 1984 a few miles southwest of the St. Thomas airport. There are five levels of decks to explore, and the ship is home to horse-eye jacks, stingrays, lobster, crabs, cup coral and sponges. The ship sits in 85 to 90 feet of water, though, so it's best for advanced divers.
Snorkelers should check out Virgin Islands Ecotours mangrove tours. You'll kayak and snorkel through the trees, which shelter juvenile reef fish of all kinds, as well as stingrays and turtles in the sea grass.
Who It's Good For: This area is great for those who are just starting out, as well as those who are advanced.
St. Croix, U.S. Virgin Islands
Of all the U.S. Virgin Islands, St. Croix is the most well known to divers. You'll be spoiled for choice with wrecks, reefs, walls and a fantastic macro dive on Frederiksted Pier.
Butler Bay, on the west end of the island, holds a number of wrecks, including the Coakley Bay, a former oil refinery tugboat. Scattered debris under the pier provides habitat for a number of sea animals, from juvenile drums and smooth trunkfish to seahorses. Cane Bay is renowned as one of the few shore dives in the Caribbean; swim out, and you'll find the wall, which plummets to 3,000 feet. It's home to soft coral, sponges and gorgonian sea fans.
Snorkelers will want to check out Buck Island Reef National Monument, which has a marked snorkeling trail that points out different underwater flora and fauna you'll see along the way.
Who It's Good For: There's appeal for all, novice snorkelers to experienced divers.
Although it's not a major dive destination, St. Maarten does offer some sites worth taking the plunge for. There are a few wrecks, and shallow reefs also make the area great for snorkelers. The old Simpson Bay Bridge is surrounded by three sunken sailboats, and, with a maximum depth of 50 feet, it's perfect for beginners. The remains of the bridge are encrusted with coral and sponges, and you'll spot moray eels, lobsters and stingrays.
Snorkelers (and any lovers of laid-back beaches, really) should make for Pinel Island, a tiny sand spit off French Cul De Sac, where you'll peel yourself from a beach chair only to refill your rum punch.
Who It's Good For: Entry-level divers and snorkelers will find this location a pleasure.
All dive itineraries in St. Kitts include the M.V. River Taw, a 144-foot freighter that sits in about 50 feet of water. It was intact until 1989, when Hurricane Hugo broke it in two pieces, but it's still a good spot to see plentiful fish.
There are wrecks, reefs, walls and caves, too. At the Black Coral Reef site, divers can check out the eponymous black coral abundant at the site, as well as creole wrasse and snapper schooling along the wall's edge. At Monkey Shoals, a large coral atoll, divers might see flying gurnards and scorpionfish, depending on the current.
White House Bay offers sheltered waters for snorkeling. While the beach is pebbly, the rocky bottom provides plenty of hiding places for fish.
Who It's Good For: St. Kitts is appropriate for those who are new to snorkeling and diving.
Passengers on an Eastern Caribbean itinerary usually only stay in San Juan for a short period of time, meaning that sites such as Mona Island -- called the Galapagos of the Caribbean for its big pelagic fish -- or Cayo Lobito on Culebra are usually out of range.
Non-divers who want to get a taste of Scuba can give it a try at Escambron Marine Park, a shallow cove and PADI training center that can get newbies down to 20 feet in just one session. This is also a good port to stay on top of the water, as Puerto Rico has some of the only bio-luminescent bays in the world. Sign up for a nighttime kayaking excursion, and marvel as your paddle muddles tiny micro-organisms that turn neon blue when disturbed.
Who It's Good For: Snorkelers who are new to the practice will feel at home there.