re: Newbie going on a cruise. Need advice.

They will nickel and dime you to death so might as well get prepared. The drink packages depend on how much you drink and the details of the package. You can usually smuggle some alcohol in shampoo bottles and other toiletries as long as you aren't obvious. We've never been caught but we haven't pushed the limit either.
The internet package is worth it if you need internet while at sea. When you get to port there are usually places that have free internet so you won't be totally cut off.

As far as staying in touch I have seen some people walking around with Walkie talkies but I have never done that so can't personally say if they work or are even a good idea.

Dive Cozumel. You won't regret it.

We normally slum it and go Carnival. They had a behind the fun tour which was awesome. It's a several hour guided tour of the ship. They take you to the engine room, bridge, galley, etc and it was fascinating to see all of that. Hope that helps. Oh and if you can get a balcony room because it gives you a place to escape if you need it.