1.) You'll be issued a sail n sign card onboard for all shipboard charges including excursions. This card is secured by either major credit card or cash deposit. CC much easier. Card also doubles as your ID. Only cash used onboard is at casino (if desired) or bingo.
2.) Suit is good enough for formal nite.
3.) Formal nite won't be first evening, probably second.
4.) Most cell phones will be quickly out of range but walkie talkies are permitted and even rented.
5.) Bar charges go on sail n sign card including tips.
6.) Cozumel is gorgeous! Very safe and definately worth seeing. I like Chanunaab Park myself, but lots of other great attractions.
7.) There's a safe in every room operated at no charge by any card with a magnetic strip on back. I've never had a problem with valuables, but good judgement should prevail.
Advice, don't miss the dinners in main dining room or shows afterwards. There's a great spot for morning coffee out the back door of Lido deck buffett either side. Plan on having a great time and booking another cruise the moment you return.