Oasis of the Seas, and its sister Allure of the Seas, are so large that the only way designers could think of to help consumers conceptualize them was to divide the vessels up into neighborhoods.

Imagine, ships so big that they can have a Central Park, with more than 10,000 living plants, plus cafés, shops and even park benches. And Central Park is only one of six neighborhoods on Allure and Oasis. These are the world’s largest cruise ships.

Similarly, Norwegian Cruise Line’s Epic features entertainment that rivals The Strip in Las Vegas. There’s even a supper club of sorts called “Cirque Dreams & Dinner,” the first show of its kind at sea under a big top.

These large megaships are so large and so complete that they could get by just fine without ports of call. They aren’t for everyone, of course, but they are ideal for families and for anyone who enjoys a variety of dining outlets and entertainment options — basically, people who enjoy the city life combined with the amenities of a world-class resort.

To get a sense of what it’s like on these ships, let’s take a quick stroll down the Boardwalk, then head over to look at Epic’s entertainment options.

Stretching for almost a third of the ship’s length, Allure (and Oasis) of the Seas’ Boardwalk is designed to resemble an old-time entertainment boardwalk a la Coney Island, with a traditional carousel featuring hand-carved wooden horses. There’s also an ice cream parlor and candy shop; a casual Mexican restaurant with a huge menu of margaritas; a hot dog stand; a Johnny Rockets burger joint; shops — and more.

But the highlight of the Boardwalk is the AquaTheater, an outdoor amphitheater ringed around the largest and deepest freshwater pool at sea, where Royal Caribbean stages amazing shows featuring synchronized swimming, acrobatics, aerialism, and incredible high diving from a pair of 30-foot platforms. You could spend an entire vacation never leaving the Boardwalk.

If Allure and Oasis were cities, they’d be New York (think Central Park and Coney Island) mixed with a little Miami. Norwegian Cruise Line’s Epic, on the other hand, would be Las Vegas.

Along with the Oasis-class ships, Norwegian Epic offers what may just be the best entertainment at sea. The headliner is the world-renown Blue Man Group. Other crowd-pleasers include the legendary comedy troupe Second City; the dueling-pianos show Howl at the Moon; great musical performances at the Fat Cats Jazz & Blues Club; the Legends in Concert celebrity-impersonator shows; and Cirque Dreams & Dinner. And of course, like Las Vegas, Epic has a casino.

Norwegian Cruise Line will soon introduce two new ships, Norwegian Breakaway due out in May 2013 and Norwegian Getaway, which begins sailing in 2014. Each will feature a suite complex called “The Haven,” which will be like a ship within a ship, with some of the biggest suites at sea, plus a private restaurant, a cocktail bar and a concierge desk.

These cities at sea have a couple of advantages over their land-based counterparts: warm sunny weather in new destinations everyday; and a day in the floating cities is almost always a better value than a comparable day in the big city.