Carly explained to Freddie that when she and Missy were younger, Missy used to get really seasick, but figures that she probably got over it since she was going on a cruise.
Freddie was the one who won, but gave it up to Missy to "protect iCarly." However, it was heavily implied that he did it for both Sam and Carly, since Sam was miserable when Missy kept trying to get rid of her as Carly's best friend and Carly was being tricked and used by Missy. This proved that Freddie really does care about Carly, Sam, and the trio's friendship.
At the end of the episode, it's shown the trip isn't even close to being as glamorous as it's advertised to be (such as being held on a cargo ship instead of a luxurious cruise ship or ocean liner). Also, Missy still suffers from seasickness, as she was seen barfing over the side of the ship and yelling, "Get me off this stupid boat!"