(MORE: Best Places to See Wildlife from the Water)

1. Cancun Underwater Museum, Mexico

Perhaps the world's most famous underwater museum, the Cancun Underwater Museum (Museo Subacuático de Arte or MUSA) is located off the coast of Isla Mujeres in Mexico's Riviera Maya, and is the site of more than 500 original, life-sized sculptures by Jason deCaires Taylor. Submerged in depths ranging from 9 to 20 feet, the sculptures offer divers and snorkelers a contemporary and cultural view of how the Mayan people have evolved through out the years in "The Silent Evolution." All of the sculptures are fixed to the seabed and made from specialized materials used to promote coral life, according to the official website.

2. Underwater Park, Green Lake, Austria

The worlds of diving and hiking collide at this park in Austria. Grüner See (which translates to "Green Lake") in Styria rests at the edge of the Hochschwab mountains. When the mountain snow is frozen in the winter, the area is used mainly by hikers since it's almost entirely dry. But in the spring and summer, when the melting snow and ice trickle down the mountain, the water rises as much as 30 feet according to the Daily Mail, submerging the park benches, trees, grassy paths, and even a wooden bridge, and giving divers a surreal landscape to explore.

3. Underwater Post Office, Vanuatu

Make your friends green with envy by sending them a postcard direct from an underwater paradise. Located within the Hideaway Island Marine Sanctuary in Vanuatu, the world's only underwater post office sits in around nine feet of water, where both divers and snorkelers are able to mail special "waterproof postcards"—international and domestic mail accepted. The postcards are then embossed (instead of stamped) with a specially-designed device. Can't snorkel down that far? Don't worry. Hideaway Island staff are available to help out.

4. Subsix Underwater Restaurant, PER AQUUM Niyama, Maldives

The Maldives is known for luxurious overwater bungalow resorts, a favorite with celebrities and jetsetters, but an attraction under the sea has also been making waves. Subsix, the underwater restaurant and venue in the PER AQUUM Niyama resort is perhaps ones of the most unique places to have dinner. Guests are taken out to to Subsix, located around 1,640 feet out into the ocean, on a speedboat, then led down below sea level via a grand staircase, according to Travel and Leisure. The restaurant can also be transformed into a venue for a wedding, set against the stunning landscape of the Indian Ocean.

Beneath the ocean at Key Largo Dry Rocks off John Pennekamp Coral Reef State Park, resting at a depth of approximately 25 feet, the 9ft-tall bronze statue Christ of the Abyss (also called Christ of the Deep) is perhaps Florida's most famous underwater attraction. The statue was donated to the Underwater Society of America in 1961 by Italian scuba entrepreneur Egidi Cressi and is the third bronze casting of sculptor Guido Galletti's "Il Cristo Degli Abissi." It is surrounded by large outcroppings and thriving boulders of large brain, staghorn and elkhorn corals as well as other reef formations, and several species of fish including a photogenic barracuda, according to the Monroe County Tourist Development Council.

6. Mermaid Shows, Weeki Wachee Springs, Florida

No "The Little Mermaid" fan will be able to resist the spectacular underwater mermaid shows at the Weeki Wachee Springs State Park, where dancing "mermaids" perform acrobatics and other underwater feats to the delight of guests watching from a 400-seat submerged theater. The mermaids, who perform with the aid of air hoses underwater, have been a tourist attraction for more than 60 years, since it was opened by former US Navy frogman Newton Perry as a roadside attraction in 1947.

7. Underwater Spa, PER AQUUM Huvafen Fushi, Maldives

Billed as the world's first underwater spa treatment space, the Lime Underwater Spa at the PER AQUUM Huvafen Fushi resort in the Maldives offers guests relaxing massages and treatments with stunning views of clownfish, stingrays and other exotic marine life. The spa's signature offering is the Lime Light Ritual, where guests are pampered with a coconut-, lime- and mineral-based treatment, followed by a massage and stay at a relaxation pod.

8. Underwater Car Wrecks, Vaersenbaai, Curacao 

The Dutch Caribbean island of Curacao is home to many amazing dive spots, but perhaps the most unique is one located in Vaersenbaai that features underwater car wrecks—classic cars and trucks from the 1940s and 50s—and cranes and construction equipment. The "Car Pile" began as an artificial reef-building project and is at depth of 60-125 feet. 

9. Shanghai Ocean Aquarium, China

The Shanghai Ocean Aquarium in China, one of the largest aquariums in Asia, houses the world's longest underwater tunnel on the planet, measuring 509 feet. The tunnel takes visitors through a coastal reef, open ocean, a kelp cave, shark cove, and a coral reef. Opened in 2002 in Pudong, the aquarium houses 28 exhibit areas for more than 10,000 sea creatures, according to Frommers. Visitors can also make special arrangements to dive in the aquarium's shark tank.