Most cruisers think about floating on top of the ocean, rather than exploring beneath it. But in many destinations, it's possible to snorkel, scuba dive, snuba (shallow diving with an air tube attached to a tank on the surface) or do a helmet dive (wearing a device with an air tube that covers your entire head, letting you breath normally while walking along the ocean floor). If "finding Nemo" is a major portion of your vacation to-do list, here are some tips on how to prepare for a snorkel or dive cruise.

Match Your Cruise Destination to Your Diving Interests and Abilities

If diving and snorkeling are key components of your cruise vacation, you'll want to be smart about which itinerary you choose to have the best underwater experience. The Caribbean offers lots of great snorkeling, but other cruise destinations are known for their water sports options, too. Pick the region that matches with your snorkeling and diving goals.

Want to see hammerhead sharks? Head for Galapagos. Want to swim through a swirling tornado of sardines? You'll find them in Mozambique. Want to explore a man-made reef composed of 60 sculptures? Dive Manchones Reef, in Cancun, Mexico. Are you interested in diving a wreck or a wall? Bali offers a great wreck in fairly shallow water -- USAT (United States Army Transport) Liberty, torpedoed by a Japanese submarine in 1942 -- and the Cayman Islands offer some awesome wall dives.

Once you've narrowed down the locations, factor in the time of year you'll be traveling. Some locations have rainy seasons, strong currents or murkier water at certain times of the year. For example, the best time to dive or snorkel Thailand's west coast is from October to May; some sites shut down in the rainy season. On the other hand, the best visibility in the Caribbean tends to be May through July.

Finally, determine whether the dive or snorkel opportunities at your top locations fit your skill level. If you're an inexperienced diver or snorkeler, or just want a low-key float, you probably want sites that aren't too deep. It may even be best for you to enter the water from a beach, rather than a boat. If you can't handle currents or low visibility, make sure your proposed dive site has calm, clear waters. Conversely, experienced divers should identify whether dives along the cruise itinerary have sufficiently challenging and interesting dives to make the trip worthwhile.

Pick the Right Cruise Ship

There are diving and snorkeling opportunities on every size of cruise ship out there. What kind of experience you want -- and what type of budget you have -- will influence which cruise ship is best for you.

For example, smaller expedition ships -- like those belonging to Coral Expeditions, Lindblad and even Star Clippers -- are often stocked with their own snorkel and dive gear. Typically, PADI instructors or divemasters sail with the ship and run exclusive dive and snorkel trips. These can be good ships for beginners, who want to give scuba a try, or those who prefer to work with the same divemaster or instructor for their entire cruise.

A smaller ship can go places larger ships can't -- plus, it's likely there will be fewer snorkelers or divers from your ship crowding the prime locations. Finally, on these ships, you won't have to stash all the dive gear in your cabin, although you'll usually be required to hang on to a set of fins, mask and snorkel for the entire trip.

Big mainstream ships offer a range of dive and snorkel shore excursions. For example, Carnival offers the opportunity to snorkel a wreck in Barbados, do snuba in St. Maarten/Martin and go helmet-diving in Cozumel -- as well as Discover Scuba shore excursions and two-tank dives for certified divers. Typically, the big ships use third-party outfits to run trips, so you'll likely be using different operators in every port. One exception is Royal Caribbean, which offers PADI dive centers on some of its ships.

You'll also want to check out each particular ship's dive program. Some might offer Discover Scuba courses for beginners, so you can learn while you cruise -- either in the ship's pool or in scenic dive spots. Others might offer refresher courses for experienced, but a bit rusty, divers. And some may give you the opportunity to get your PADI certification. Royal Caribbean's PADI dive shops offer all of the above -- as does Paul Gauguin Cruises and Star Clippers.

Compare Ship Tours with Independent Operators

Some highly experienced divers prefer to organize their own dives with local dive shops in various ports. That's because ship or shore excursion dives are often slanted toward less experienced or intermediate divers. If you're making your own arrangements, be sure to gather lots of online intel on any providers and read reviews. You'll also need to be sure that the dive trips match up with your time in port, and that you'll be returned to your ship with plenty of time before departure.

Check on Age and Health Restrictions

Scuba divers will have to fill out detailed medical forms and waivers before taking the plunge, and some countries require that you've had a dive medical assessment within the past two to three years. If you've had any medical changes since you last dived, including new medications, you should check with your doctor before diving. Most dive programs prohibit people with heart conditions, diabetes, epilepsy, asthma, high blood pressure, walking disabilities and women who are pregnant from participating.

Age can also be a limiting factor, regardless of health. We've seen diving shore excursions with a minimum age of 15 and maximum age of 60. In French Polynesia, many dive shops require a medical certificate for divers over 75. For helmet dives and snuba, the minimum age is often eight. If your group of water sports enthusiasts includes kids or retirees, make sure the available tours are accessible to them.

Some dive trips have minimum or maximum weight limits, too.

Get Certified

If you're considering taking a Discover Scuba course, at home or during your vacation, we strongly recommend that you do it in the ocean on a shore dive, rather than in a swimming pool. (For example, Royal Caribbean does its initial dives in the ship's pool.) We've done both, and there's a world of difference. In the ocean, you're delighted and distracted by all there is to see, which makes the whole process easier and less scary. In a pool, all you can think about is how weird it is to be breathing underwater.

If you're certified, but haven't dived in a while, you may want to take a refresher course prior to your trip. That way, you'll be ready and confident. Some ships also offer refresher dives. If you haven't dived in more than two years, you may be required to complete a refresher course before participating in dives.

Regardless of whether you're newly certified or a longtime diver, don't forget your PADI certification card. If you need a replacement card in a hurry, you can get a PADI eCard instantly online. Also, make a copy of the last page of your dive log and bring it with you.

Decide Which Gear to Bring, Borrow or Rent

Confirm exactly which gear the ship or shore excursion provides. For example, Royal Caribbean requires you to provide your own mask and snorkel for diving -- even though they supply the rest of the gear. On the other hand, Star Clippers furnishes all snorkel and scuba equipment. You might also be able to rent gear at local shops.

Whether you're snorkeling or diving, a good mask is key. Even if your ship provides masks, you may want to bring your own, for a perfect fit (or to find one with prescription lenses). If you have a new mask, the pros recommend gently cleaning it with toothpaste before you use it to remove the coating that may cause it to fog up.

You may want to bring your own snorkel, too, if you don't like the idea of using a mouthpiece that was in someone else's mouth. Best to leave the fins at home, though, because they're a pain to pack. If you're bringing any gear of your own, be sure to get it out and test it well before your cruise, so you have time to replace anything that's not in good shape.

Note that equipment you rent or borrow might be different from what you're used to, due to regional differences. For example, European pressure gauges measure pressure in BAR, not PSI. Be sure to go over equipment with your divemaster or rental shop to make sure you understand any differences from what you're used to.

Pack Smart for Your Trip

Gear isn't limited to what you wear. You might want to bring an underwater camera or an underwater case for your digital camera to snap selfies with sea creatures during your outings. Don't mess with photography if you're just learning to dive -- you'll have enough to focus on without that distraction -- but snorkelers or more experienced divers will enjoy grabbing some photo ops. It's a good idea to have a waterproof pouch for your phone if you're taking it along on the dive boat, too.

Other items for your packing list include coral-friendly sunscreen (you don't want reef-destroying oil or chemicals), ear drops to dry up water in your ears, water shoes, hair-ties to keep long hair from getting tangled in your mask and dive skins/rash guards (we like to wear these without wet suits to protect against sunburn), or long-sleeved swim shirts with SPF protection (particularly important to cover your back when snorkeling).

Checking the Cruise Critic message boards, divers are all over the map as to whether you should bring a diving knife with you (in checked baggage when flying, of course). Some people have had no problem bringing them on ships, while others have had them confiscated when not diving.

Consider Dive Insurance

Nobody likes to think that something could go wrong -- but it's good to be prepared. Most travel insurance policies don't cover injuries or accidents from scuba diving. Take a close look at your policy and think about getting special dive insurance.

Study the Dive Area

It's a great idea to study the local marine life where you'll be snorkeling or diving before you head underwater. Consider getting an identification card and learning some of the hand-signals for identifying the most common species (our favorite hand-signal: parrotfish). Is there anything dangerous where you'll be going? Probably a good idea to find that out, too.

You'll also want to get the lay of the land. If you're snorkeling from land on your own, check to see if there are times of day when the prime spots are crowded. For example, off the west coast of Thailand, day-trippers invade snorkel spots in the late morning and early afternoon.

Plan Your Flights Around Your Dives

When you're underwater, your body experiences more pressure than at the surface. In an airplane, the pressure is less than on the ground -- so, considerably less than underwater. That can cause some problems if you hop on a plane right after a dive.

The Divers Alert Network and the Undersea and Hyperbaric Medical Society have guidelines for how long you should wait between diving and flying. The minimum suggested waiting time for a single, no-decompression dive is 12 hours. So, if you schedule a dive for the last full day of your cruise or on debarkation day, make sure you add an appropriate buffer before your flight home.