Sea kayaking is one of the best shore excursions available from a cruise ship. It puts you at water level in places that are inaccessible by any other means -- inside sea caves, through mangrove tunnels, along cliff faces, and across bays that look completely different from six inches above the surface than they do from a ship deck.

The barrier to entry is low. Most guided kayak tours are appropriate for beginners, require no prior experience, and provide all equipment. The physical demand is moderate -- a two-hour paddle covers three to five miles and requires basic upper body fitness, but nothing that would exclude most adults.

Here is where to find the best kayaking excursions at major cruise destinations.

Alaska offers the most dramatic kayaking scenery in the world. The Inside Passage -- the network of channels, fjords, and islands between the mainland and the outer islands -- is protected from Pacific swells and offers calm water for paddling even in variable weather.

Sitka Sound is the premier kayaking destination in Southeast Alaska. The harbor islands east of town provide protected water for beginners, while the outer sound offers more challenging paddling with better wildlife viewing. Sea otters, harbor seals, bald eagles, and humpback whales are all common sightings.

Ketchikan offers kayaking through the Tatoosh Islands and Orcas Cove. The protected channels between the islands are ideal for beginners, and the wildlife -- Steller sea lions, eagles, seals -- is abundant.

Juneau has kayaking in Auke Bay and around the Mendenhall Glacier. Paddling near the glacier face, with icebergs floating in the water around you, is an experience that is difficult to replicate anywhere else.

What to expect: Water temperatures in Alaska run 45 to 55 degrees Fahrenheit even in summer. Reputable operators provide dry suits or wetsuits. Do not book a kayak tour in Alaska without confirming that thermal protection is included.

The Caribbean offers a completely different kayaking experience -- warm water, clear visibility, and some unique natural phenomena.

Mosquito Bay, Vieques (Puerto Rico) is the brightest bioluminescent bay in the world. Dinoflagellates in the water produce blue-green light when disturbed, and kayaking through the bay at night turns every paddle stroke into a light show. The effect is most dramatic on moonless nights. Tours depart from Esperanza and run about two hours.

Bioluminescent Bay, Fajardo (Puerto Rico) is accessible from San Juan and offers a similar experience to Mosquito Bay. Night kayak tours are available from multiple operators.

St. John, USVI has excellent kayaking in Trunk Bay and the protected waters of the north shore. The water is clear enough to see the reef below your kayak, and the snorkeling at the end of most tours is exceptional.

Belize offers kayaking through mangrove lagoons and along the barrier reef. The mangrove ecosystem is extraordinary -- the roots create a nursery habitat for juvenile fish, and the channels between the mangroves are navigable by kayak but not by larger boats.

What to expect: Caribbean kayaking is warm and relaxed. Most tours include snorkeling stops. Bring reef-safe sunscreen -- many Caribbean destinations have banned oxybenzone-based sunscreens to protect coral reefs.

The Mediterranean coastline offers kayaking through sea caves, along dramatic cliff faces, and into hidden coves that are inaccessible by larger boats.

Dubrovnik, Croatia has excellent kayaking along the city walls and around the Elafiti Islands. Paddling beneath the medieval walls of the old city, with the Adriatic stretching to the horizon, is one of the great urban kayaking experiences in the world.

Kotor, Montenegro offers kayaking in the Bay of Kotor, a fjord-like inlet surrounded by mountains. The water is calm and the scenery is extraordinary.

Santorini, Greece has kayaking along the caldera cliffs -- the sheer volcanic walls that drop into the sea. The scale of the caldera is difficult to appreciate from above; at water level, it is overwhelming.

Cinque Terre, Italy offers kayaking between the five villages along the Ligurian coast. The sea caves and arches along this stretch of coastline are accessible only by kayak or small boat.

Experience level: Most tour operators rate their tours as beginner, intermediate, or advanced. Be honest about your fitness level and prior experience. A beginner tour in calm water is more enjoyable than an intermediate tour that exceeds your abilities.

Duration: Two-hour tours are appropriate for most cruise passengers. Half-day tours cover more distance and offer better wildlife viewing but require more time and physical effort.

Group size: Smaller groups (eight to twelve people) offer a better experience than large groups. Ask the operator about their maximum group size before booking.

Equipment: Confirm that the operator provides all necessary equipment including paddles, life jackets, and appropriate thermal protection for the water temperature.

Weather policy: Ask about the operator's cancellation policy for weather. Reputable operators will cancel or modify tours in unsafe conditions and offer refunds or rebooking.

Book in advance: Popular kayaking tours sell out weeks ahead, especially in Alaska during peak season (June through August) and in the Caribbean during winter.

Independent vs. ship excursions: Independent operators typically cost 20 to 40 percent less than cruise line excursions. The risk is that if the tour runs late, the ship will not wait. Book with a generous buffer before all-aboard time.

Physical requirements: Most kayaking tours require participants to be able to enter and exit a kayak from a dock or beach, paddle for the duration of the tour, and swim if necessary. Check the operator's requirements before booking.

Kayaking is the shore excursion that most cruise passengers wish they had booked. It is active, immersive, and puts you in places that no other excursion can reach. If your itinerary includes a port with good kayaking, make it a priority.