Ask yourself what is at the top of your bucket list. Would you rather hike a glacier, or a rainforest? Seeing rare animals in their natural habitats is a huge part of the appeal of expedition cruising. 

A trip to the Galapagos is a exciting for many animal lovers, since it is home to species not found anywhere else on Earth like marine iguanas and blue- and red-footed boobies. If you seek howler monkeys, pink dolphins, and toucans, head down to the Amazon. Expedition also head to colder, ice-bound regions that the typical ocean ship can't navigate, such as the Arctic and Antarctica. Cruises to these can offer the chance to see penguins, polar bears, and walruses.

Also consider if you really have the time to do the trip. Needless to say, these are not quick getaways; Antarctica itineraries are often 21 days long, while Galapagos trips are typically seven to 10 days, including some time on land. 

Expedition ships tend to be much smaller than typical ocean-going vessels, and these cruises are not geared toward those looking for a party atmosphere or casinos and over-the-top shows.

"Large ships would not be able to operate in the areas the expedition ships go to," Steinman says. "Because of this, there is limited space onboard for anything other than a restaurant and lounge." 

That doesn’t mean you’ll be roughing it, though. While not a luxury line, Hurtigruten has ships traveling to Antarctica, Greenland, and the Arctic that include onboard extras like fitness centers, hot tubs, and saunas. Lindblad Expeditions and Quark Expeditions also provide creature comforts, with ships ranging from rustic schooners to purpose-built vessels with strengthened hulls designed to handle ice. 

Celebrity Xpedition, a 100-passenger mega-yacht based in the Galapagos, is more high end, with multicourse dinners and all-inclusive fares that cover drinks and Internet. And Silversea's decadent 130-passenger Silver Explorer was built to break through the icy waters of Antarctica while you dine at the Relais & Château restaurant.

Peak season in the Arctic is June through September, when the sun shines 24 hours a day — the polar bears are active and hunting, the birds migrating, and the flowers blooming. December to March is the best time to spot humpback whales in the Sea of Cortez, and mid-September to mid-November is the season to see migrating seabirds and spawning salmon in the Pacific Northwest. One exception is the Galapagos, where ships like Celebrity Xpedition, National Geographic Endeavour, and National Geographic Islander sail year-round.

Expedition cruises are all about getting off the ship for long days of exploring, so physical fitness is important. Expect active days venturing ashore in Zodiacs, kayaks, or canoe launches. Daily excursions, which are often included in your fare, include things like swimming, snorkeling, hiking, and plenty of walking. A Hurtigruten expedition along the coast of Norway could also give you the opportunity to go dog sledding and snowmobiling. While the days are packed, the evenings are quiet, with the big excitement being expert lectures in the lounge.