Like marriage, your vacation is often a series of compromises. That is never more certain than when picking a cruise.

Will it be a megaship, carrying thousands of passengers but offering a spa, swimming pools and multiple dining choices? Or will it be a cozy vessel taking fewer than 100 passengers, without workout facilities and with just one dining room – but a vessel that will nuzzle the glaciers in Alaska or put you in motorized rafts to reach beachfront rain forests?

Will it be a big-name line, with lavish musical revues, comedians and a string quartet? Or will it be the little-known brand that dispenses with entertainers in favor of PBS-style documentaries and power-point presentations by its naturalists?

Do you need a hair dryer and flatscreen TV in the cabin, or can you get by with a shower stall that contains your toilet?

Long gone are the days when cruising mainly appealed to the “newly wed and nearly dead.’’ While folks in those demographics still come onboard, the vast majority of cruise ships are aimed at luring the 90 percent of American adults who have never been on a cruise.

Most fleets are designed to appeal to the middle class – with sparkly things such as rock-climbing walls, wave pools, shopping arcades, specialty restaurants ranging from Tex-Mex to sushi, and fitness areas whose dozens of machines face floor-to-ceiling windows on the ocean.

Yet other cruise lines aim for the deeper pockets of better-educated, better-traveled passengers. These ships may have planetariums, onboard acting troupes, top crystal and china in the dining room, and a visiting faculty of noted lecturers.

And then there are the small-ship lines that trade the geegaws and Broadway-style revues for basic comforts and destinations to the exotic — or at least the undeveloped. Typically a fraction the size of mass-market ships, these “soft-adventure’’ vessels appeal to those wanting first-hand experiences, not to frolic on “cruises’’ but rather to see what else is out there.

The counterpart of these slightly roughing-it excursion ships are those that also carry fewer passengers but emphasize luxury. Lavish staterooms replace tiny cabins that may not even offfer a chair. The bar in each stateroom is stocked with beverages the occupants have requested before boarding. There may be butler service, and the dining room menus promise mini-feasts.

As with most other things, you get what you pay for. The mass-market, 2,500+ passenger ships profit from volume purchasing. Per diem fares on these ships can be less than $80. Excursion ships, on the other hand, can easily cost five times that, and luxury vessels, much more.

So before you head for your first, or fifth, cruise, consider what else is out there, and what it costs.

To learn more, consult the web site of Cruise Lines International Association (www.cruising.org). This trade organization represents 25 lines with more than 97 percent of passenger capacity leaving North American ports.