Fly Aweigh is Carnival's cruise air program. You do not have to pay extra if you don't use it. I've been on seven Carnival Cruises with an eighth booked (four I drove to and four I flew to). I would never use cruise air - from any cruise line. First of all, with a little work, I have been able to beat their prices. Second, and much more important, is the flights they choose for you are not what I would choose, and missing the cruise because of bad flight planning is a possibility.
If you have a credit card you can use that for your on board spending. That way you don't have to worry about how much to bring for your on board account. Since you cannot spend cash on board (you have to set up an on board account) I strongly suggest using a credit card for that purpose (over a debit card or cash). You can check your on board spending via the interactive TV in the cabin.
Spending money on shore is just like spending money on a vacation in an American city. How much would you spend if you went to Key West? How much would you spend if you went to New Orleans? I have no idea what your spending habits are, or what you might want to buy on shore.
All the food you need is free. However, you can purchase specialty food items if you want. Either the Supper Club ($30 per person for dinner) or if there is no Supper Club on the ship, then you can purchase an upgraded steak. I believe that is $15 or $18 per steak. Hard and soft drinks are extra, as is bottled water. Water, regular coffee, milk, juice and punch are all free (specialty coffees, think Starbucks, cost extra). If you order room service the food is free, however, the person bringing you the food will expect a tip ($1 to $2 depending on the order). Otherwise, food in the main restaurant (aka MDR) and the buffet is free.
Photographs are expensive. You have the option to buy after you've seen the picture. There is a souvenir shop on board. Tips are $10/day/person. Tips are automatically added to your on board account. My on board account for my last cruise (eight nights) was $442.34. I don't gamble and I had three drinks. I bought a few photographs and some souvenirs. For the prior eight night cruise my on board account was $208.64. I didn't purchase any souvenirs. So, as you can see, two cruises, three months apart, and more than a 100% difference in the amount spent on board. For my third cruise (one week back in May 2004) my on board account was over $1,000.
If you purchase every picture the ship photographers take of you and your family, then I hope Bill Gates is your uncle and your are his favorite relative. 8x10s run $22 and the smaller ones are about $12. The ship photographers will take pictures of you when you board (embarkation photo), while you are on the ship (sail-a-way), when you get off the ship in port, at dinner, on formal nights, plus you can set up special photo sessions if you want. A family of four can easily end up with forty photographs running $700. You don't have to buy them all. I spent about $330 on my nine night Princess cruise (which was my first Princess cruise) just for photographs. And I didn't purchase all the photographs (though in hind sight there were a few I did purchase that I should not have). You have to be careful about buying the photographs. I suggest taking your own camera. The pictures you take are a lot less expensive (I easily take 100 photographs a day).