sky class (the mid-tier) should get you a cabana on the suite beach on labadee, but it might not be one right over the water. in addition to the over the water ones, there are beach cabanas, hillside cabanas, and standard cabanas.
star class (upper tier) would give you a better chance of an over-the-water cabana, if that's what you want. there are only 6, though, so it's not guaranteed - i wouldn't book star class just for that.
sea class are "junior suites", which have some basic perks - double loyalty points, usually a walk-in closet, suite restaurant for dinner on some ships, and the room is bigger than a balcony - but no concierge service or other onboard perks, and no access to the suite beach.
if you are going on a newer ship (oasis class or quantum class) there is a real difference between sky and star. on the other ships, not so much. royal just did the division into sky and star tiers fairly recently and started with oasis and quantum class, and it hasn't really spread to other ships yet. on all ships sea class/junior suites have the very limited benefits, though. but if on an older ship, you might have an ok chance of an over the water cabana even in sky class; i haven't been in a suite on an older ship since they instituted the new system, so i'm honestly not sure.
just a heads up, if you have been in a cabana on castaway cay, the royal cabanas are nothing like that. they are tiny huts with 3 walls, a roof, a ceiling fan, and a deck area. there is a couch inside, a couple of loungers on the deck, and they give you a cooler with water. sometimes you get a couple of floats. that's about it. no fridge, no hammock, no doors/windows, no safe, no changing area, etc like on castaway cay. we used to get them when they were $250 for an over the water one, but now they are usually $500 and we don't think it's worth it. the other cabanas are cheaper, and plenty of people like them, they're just not our cup of tea - i want to be on the water!
also, this info is for labadee. if you're asking about coco cay, i'm not sure how that works since the recent split into star and sky class.
