MIAMI — When Miami middle school teacher Kathy Balboa boarded the Norwegian Sky in December, she expected to find the same amenities she had enjoyed on a previous voyage on the new Norwegian Getaway: water slides, varied entertainment venues, a Vodka Ice Bar and onboard texting capabilities between family members.

On her December voyage, those features were nowhere on board.

The problem: Norwegian Sky was launched in 1999, before water activities and comedy shows became standard cruise ship features. Though it was refitted as the Pride of Aloha in 2004 and then redesigned for the Norwegian fleet in 2008, those state-of-the-art amenities won’t come until February 2017.

“It’s like going out with a supermodel and then going out with the ugliest girl in the world,” Balboa said. “I don’t want to cruise for a long time because I was so disappointed.”

The disconnect between old and new ships explains why Norwegian and other major lines are spending hundreds of millions of dollars over the next two years to refresh their vessels. In doing so, they create brand consistency, ensuring that cruisers find the same features and equivalent experiences no matter which of the line’s ships they are on. And even the most extreme makeovers come at a fraction of the time — and cost — of building a new ship.

For those in the industry, Norwegian’s announcement late last year that it would spend $400 million in ship renovations over the next two years was the continuation of a familiar trend that picked up steam a half-dozen years ago. Renovations helped better meet consumer expectations and created more profit for lines. As new ships grew bigger and incorporated flashy new features, lines were forced to shift from traditional cosmetic touch-ups of older ships — such as new carpets and bar menus — to complete overhauls.

Perhaps the most extreme example was the 2013 near-gut of the Carnival Destiny.

Carnival Cruise Line radically transformed its decades-old Carnival Destiny, adding nine new dining options — including four specialty restaurants — six bars; a recreation complex with a ropes course, mini-golf and basketball court; and its crowning glory: a water park with a 334-foot-long Twister slide, the longest in the Carnival fleet. The renovation added 182 staterooms, taking the ship from a 2,642-passenger vessel to one with capacity for 3,006 guests. The ship even debuted under a new name, the Carnival Sunshine.

The tab: $155 million, plus 75 days out of service — a bargain when compared with Carnival’s new $780 million Carnival Vista. That 3,936-passenger ship, due to launch in May, took two years to build.

“The revolution is in the renovation,” said Mike Driscoll, editor of the trade publication Cruise Week. “You take the best of your old ships — when it came out Carnival Destiny was the best Carnival ship — (and) you spend the money to make it good for the modern era.”

The challenge — and opportunity — are in the numbers. More than 60 percent of ships in service in the three major lines — Carnival Corp., Royal Caribbean Cruises and Norwegian Cruise Line Holdings — are more than 10 years old.

Between Jan. 1, 2015, and Jan. 1, 2017, about 20 percent of all ships, 34 total, are slated for extreme makeovers that include the addition of new entertainment venues such as a Beatles-inspired club on the Norwegian Epic; new restaurants including SHARE by Curtis Stone on the Emerald Princess and Ruby Princess; and new attractions like the two racer slides debuting on Liberty of the Seas. In that same time frame, more than a dozen new ships will join fleets across the world.