No question is silly. [img]/infopop/emoticons/icon_smile.gif[/img] A gown isn't necessary for formal night, nor is a tux; a nice dress, like you might wear to an evening wedding, and for men a suit or matching (dark) sport coat & slacks will be fine. YOu can certainly bring aboard a small water bottle, but they will probably frown on the six-pack! Tap water onboard is good, and you can just refill your bottle from there as often as you need. Sodas are relatively expensive; if you drink lots of pop, you may wish to purchase a drink card when you arrive, if they are available for adults (I am not sure about these on short cruises), as it will save you some of the cost. Juices, including lemonade, are included in the price and available 24/7, which I found was a great option! And don't worry about your passport - as long as it's valid, it should be fine. If you have changed drastically (hair color, a big weight difference, etc.) you may want to have your current driver's license available as well, but I doubt they will question the passport.
My best advice - relax, go with the flow and enjoy! Have a great time! [img]/infopop/emoticons/icon_razz.gif[/img]