The idea probably came up when the producers of "The suite life of Zack and Cody" couldn't think of a new cheesy pun or idea for an episode, so they thought of moving the alleged twins Zack & Cody (Dylan & Cole Sprouse) to a cruise ship, hence the suite life on deck.
It stars the same characters as the old show, except they replaced Maddie Fitzpatrick with a girl named Bailey (Debbie Ryan) who's apparently from the country even though she hasn't got a hint of country accent in her voice. They call her a "country bumpkin", though she says "you guys" instead of "y'all" like most country people (trust me, im from Alabama.)
It's basicly the same thing as the old show, except they're on a cruise ship and they go different places.
SLZ&C Producer #2: Well, I just got back from a cruise ship, and it would be a great place for D & C to ensue shananigans.
SLZ&C Producer #3: *great* idea!!!
SLZ&C Producer #1: I know!
And so the Suite Life on Deck was born.