I can answer #4 and part of #1...
1) Your free drinks will include coffee, tea (hot and iced, several flavors of hot), juice, and milk. The soda card is supposedly per person, but on a 1-week cruise you'd have to drink about 3 sodas per day to make it worth the money. Their "bucket of beers" is something like 5 beers (Corona, I think) for the price of 4. All 5 bottles are shoved in a little bucket and covered with ice.
4) You'll have a great time on the scuba certification. As I remember, you get a CD and/or book in advance and study at home. When you board the ship, you start doing your pool work on the ship. You'll do 4 certification dives during the week, probably two in Cozumel and two in Grand Cayman, and have your certification when the cruise is finished. I would suggest buying your mask, fins, and snorkel in advance...especially the snorkel, which probably isn't sterilized between different users.
My son and I got our certification in March on a Dawn Princess cruise, but we didn't use their program...we did our class and pool work at home (Kansas City), then scheduled our cert dives in Cozumel and Grand Cayman. All 4 dives were "shore dives". In Cozumel, the bottom features were not particularly exciting but we did see some eels and a puffer fish. The shore dives in Grand Cayman were awesome due to the large rocks and caves. There, we saw a sea turtle and barracuda.
Photos and a writeup are on my web site at www.mycruiseadventures.com