Americans may not need a passport if sailing domestically or round-trip from the same U.S. port — even if the cruise anchors in foreign ports such as Bermuda or Cancun. The “closed-loop cruise” loophole only works in the Caribbean, however, and it’s not valid for all the islands. Be sure to check entry requirements with your cruise line. Passports can take several weeks to obtain, so be sure to plan ahead.

If you already have a passport, the expiration date must be at least six months from your travel dates (the exact amount of time it needs to be valid depends on the countries you’re entering). You won’t be allowed to board your flight or ship if the requirements aren’t met, so check the dates well in advance of your departure. You can enlist the help of the government’s rush service or a private expediting company if you need a renewal at the last minute.

While many countries don’t require a tourist visa for American citizens or offer an on-arrival visa service at the airport (for example, Vietnam), some destinations such as China, Brazil, and Russia require travelers to have their visas in place prior to entry, no matter what means of transportation. Be sure to check the U.S. Department of State web site to cover your bases, and apply in a timely fashion.

Check the weather reports of countries you’ll be visiting. This piece of advice may seem obvious, but you’d be surprised by how many people pack based on their own biased notions. For example, in July — when it’s warm in many places — it’s actually chilly in areas such as Scandinavia and San Francisco. Likewise, not all of Asia is warm and toasty year-round. In January, the weather in Hanoi and Hong Kong can be nippy, and it’s best to pack sweaters, jackets, and scarves to wear in layers.

Sure, you may think that your toddler niece or nephew is adorable, but imagine being on a ship with a 1,000-plus of them. If you prefer a vacation that’s more adult-oriented than family-friendly, avoid big-ship cruises during the summer and holiday weeks like Christmas, when throngs of children are out of school, and onboard.

Cruise pricing can be all over the map, based on timing (early-booking discounts and last-minute deals are common), your travel agent (some agents can pass bulk discounts onto you), and simply sheer luck. Don’t spoil the mood at dinner by bragging about the too-good-to-be-true promotion you snagged. It’s poor etiquette, and nobody wants to feel sore about what they hear.

If you’re prone to motion sickness or seasickness, avoid booking a cruise in notoriously choppy waters. Transatlantic crossings in the winter can be particular rough, as are passages through the Tasman Sea, between Australia and New Zealand; and the Drake Passage, between South America’s Cape Horn and the South Shetland Islands of Antarctica.

Sharing a cabin with a friend or relative can be a great way to save money and to rev up the good times. However, compatibility is key. Maybe you don’t appreciate that your cabinmate snores or that he or she isn’t as tidy as you. If that’s the case, then your vacation could turn into a cruise from hell — even more so if you’ve booked a small cabin.

Cruise ship spas are often respites from the crowds. But most treatments err on the pricey side, and some — such as slimming treatments — promise results that don’t live up to their claims. Think twice before getting swayed by sales pitches and shelling out hundreds of bucks with little to show for it.

Unless you’re not planning to leave the ship or you're signing up only for the ship's tours, you’ll need some money in port — especially for those small towns in Europe and parts of Asia where many establishments don’t take credit cards. Instead of spending your very limited time onshore scrambling to find an ATM or a change bureau, it’s handy to have some local currency on hand, which you can get on your pre-cruise night or from your bank at home. Although some cruise ships may have some for exchange, it’s wise not to count on it.