Backstreet Boys Dress Up as the Spice Girls, Sing ‘Wannabe’ & Cover *NSYNC -- Watch!

It was one big '90s party as the boy band set sail from Miami, Florida, for their sixth fan cruise on Thursday.

Having just celebrated their milestone 25th anniversary, the Backstreet Boys have reinvented themselves -- as the Spice Boys!

The boy band hit the high seas for their sixth -- and perhaps most FOMO-inducing -- fan cruise on Thursday and when it came to Sunday's "Party Like It's 1993" bash, they emerged dressed up as the Spice Girls.

Thousands of fans on board the Carnival Sensation cheered as the guys hit the stage with Nick Carter as Emma Bunton, Howie Dorough as Geri Halliwell, Kevin Richardson as Victoria Beckham, AJ McLean as Melanie Brown and Brian Littrell as Melanie Chisholm.

And, if that wasn't enough, the quintet also pulled off their spiciest moves to the girl group’s smash, “Wannabe,” with Littrell holding a glass of wine while incorporating some workout moves as a salute to Chisholm, who’s also known as Sporty Spice.

Carter, meanwhile, bopped his beautiful ponytails and pink manicure to the beat while channeling Bunton, who was his fellow judge on last year’s ABC series Boy Band.

“We were surprised how amazing they looked as women -- legs for days. I’m looking at you, Nick Carter!” cruiser Chrissy Schulz told ET. “We were sure they must have been practicing walking in those shoes because that and their girl power attitudes were amazing!”

"A lot of fans have come to expect Nick to dress like a woman at least one night on the cruise, but to see them all doing it was hilarious," added Lori Meono, who has done all six cruises. "I almost died with laughter when they first came out. The first thing I noticed was how comfortable Nick was. He was rocking those platforms as though they were his favorite pair of sneakers and Kevin's 'Posh' attitude was perfect -- I've never been so happy to have someone turn up their nose at me!"

Sailing away from Miami, Florida, with the group dressed up as pirates, the nonstop party commenced with Thursday’s Double Trouble Game Show event, followed by a “Fast Times at Backstreet High” bash.

Throwing it back to high school days, the group covered fellow boy band *NSYNC’s hit, “Tearin' Up My Heart.”

While Carter dressed as a nerd and rocked the drums, Littrell serenaded his gorgeous wife, Leighanne -- who was dressed up as a cheerleader -- while down on his knees. The cute couple also stole the show with their rendition of Britney Spears’ “Baby One More Time.”

In between blackjack tournaments, storyteller sessions, quiz nights and hilarious sumo wrestling showdowns, Carter attempted to don a disguise and play hide-and-seek with fans, which resulted in an epic fail.

Friday night, the party boat celebrated “We’ve Got It Going On” night with Carter dressed as Thor.

But it was Saturday’s “Millennium Night” which wowed cruisers as the quintet brought back their all-white outfits and performed their entire 1999 smash record, Millennium. They then moved on to hits from 2000’s Black & Blue and 2005’s Never Gone, as well as solo songs like McLean’s “Mr A” and Richardson’s “Nobody But You.” Even a track from Nick & Knight -- Carter’s collaborative project with New Kids on the Block’s Jordan Knight -- made the cut.

McLean and Carter then jumped into the fan-filled pool, while Dorough rocked the drums and Richardson and Littrell continued prancing around the stage.

However, it was the Spice Boys cameo that was the talk of the boat. Rumors of a potential collaboration or joint tour between the two pop powerhouses have circulated for years, but in October, the group told ET that there were no immediate plans to unite.

The boys also dished on their former Spice Girl crushes and partying with the girls back in the day. McLean shared that he thought Beckham and Halliwell were “so gorgeous and hot,” and that Dorough “talked about Baby Spice all the time.”

“We were young and single, so I used to catch up with them at festival shows and go partying,” Dorough explained. “We used to go to this place, Momo, a Moroccan food place, and downstairs they have music and a band. Baby Spice was reminding me today about the last time we were all in the U.K. and she was like, ‘Last time we were together, we were salsa dancing downstairs.’ There must have been some alcohol consumed because I don’t remember this too much!”

“But they’re great girls, we respect them a lot and we’d love to do something with them in the future if it all works out,” Dorough continued. “It would be a fun collaboration.”

See more on the group, who are preparing to drop their new single later this month then resume their Larger Than Life residency in Las Vegas, Neveda, below.