Vasco da Gama had it right: Water-based exploration is the way to go. Even today, many places worthy of discovery are more accessible by sea than by any other means. Adventurous dim-dims can follow Tom Huth's example and visit the islands of Papua New Guinea aboard Orion Expedition Cruises' MV Orion; 11-day sails depart Cairns in the spring and fall (61-2-9033-8777; orioncruises.com.au; $7,221–$8,804 per person). With the help of experts, we have chosen cruises in six regions where the experiences are rare and rewarding.

Unless otherwise noted, rates are based on double occupancy. Airfare to the departure port is not included. Prices quoted are for January 2008.

Alaska
The narrow straits of Canada's Alexander Archipelago are no problem for American Safari Cruises' 22-passenger Safari Quest. The eight-day sailing departs Juneau every two weeks, docking at small ports including Glacier Bay National Park, a 3.2-million-acre behemoth with thousands of floating glaciers. Inflatables and kayaks weave through Glacier Bay's fallen iceberg mazes in search of puffins, sea lions, bears, moose, eagles, and other animals that emerge in summer. Beyond the Icy Strait (look for humpback whales) is tiny Elfin Cove, a fishing village on Chicagof Island with a population of 32. Continuing down the Chatham Strait, the ship sails through waterfall-laced fjords, stopping for nature hikes and a hot springs soak (206-284-0300; amsafari.com; $5,495–$8,295 per person).

Amazon
The Amazon River's rain forest contains a third of the world's wildlife species, and its watershed has the highest biodiversity concentration per square mile. International Expeditions' ten-day Amazon Voyage brings adventurers to the region aboard 45-passenger riverboats. Departing from Iquitos, Peru, the boats sail upriver, affording views of marsh birds, monkeys, and other rain forest dwellers along the way. Gray and pink river dolphins swim beside the ship, and smaller excursion boats weave through the wetlands so you can spot river otters, endangered scarlet macaws, and the distinct plant life of a flooded forest. You may encounter colorful poison-dart frogs, but not to worry: They're only dangerous when eaten. The boat docks at villages along the Amazonian coast, and a hike across the Terra Firma ("High Ground"), an elevated path with a vaulted canopy that winds above the rain forest, allows another perspective on the jungle (205-428-1700; ietravel.com; $3,048–$3,298 per person).

Antarctica
The austral summer months (Nov.–March) mean long days and moderate temperatures, making it the best time to explore this remote region. Polar Cruises offers an 11-day adventure for 110 seafarers aboard the Ioffe or the Vavilov, ice-strengthened research vessels with a few added creature comforts, such as a sauna and a bar. After crossing the Drake Passage, the ships stop at the South Shetland Islands for sightings of fur seals, albatross, humpback whales, and Adelie penguins (which have a large rookery here). An active volcano on Deception Island has formed a warm-water crater where hardy travelers can have a refreshing swim while icebergs glide past the misty caldera. Real thrill seekers can even camp out on the Antarctic mainland. Some itineraries include an extra day of kayaking for a different experience of the icy wilderness (541-330-2454; polarcruises.com; $6,750–$10,450 per person).