This really depends on what you are planning to do. I have pasted my answer to a similar question below. Keep in mind, the person that was asking didn't drink, so that can make a substantial difference in how much you should take.
So if you are planning to drink, figure anywhere from about $4 to $12 for drinks, and a 15% gratuity that is automatically added. You can buy some tickets in advance that you can use in the bars and you can also buy some wines and other things in advance, so you will need less money while on board. Keep in mind that soda is also an extra expense.
It really depends on what you already have included in your cruise. As well as what you plan on doing on the boat and excursions. So you will be the one that is best able to determine your budget. You can also contact your travel agent, they will be able to give you a better idea of how much you will need.
Hope this helps!
Here is my answer:
Since you don't drink that will save you a lot of money. However, how much you will actually need to take depends on how much of the involved costs you have already paid. Whenever I go on a cruise I try to have as much paid before hand as possible. Including tips, etc. This will prevent you from having to take several hundred just for the tips.
I find on my cruise that there is very little more I need to purchase as far as food is concerned. As a matter of fact the only other food item I purchased while on board was an iced cookie. I was so stuffed that I really didn't want anymore to eat. Also, sodas and other drinks were included, so I didn't have to pay extra for that.
Alot of cruises want to create a cashless environment. What I did on my last cruise is give them the credit card I would like to put purchases on. Then I just used my room key to make charges and never had to have any cash. You should look into travelers checks or the credit card that acts as travelers check. While on the cruise itself it is very secure to use your credit card, no worries there. But you should be a little more concerned on the island. Travelers checks are the safest way to go as far as this is concerned.
You need to take into account what tours and stuff you will want to do. Again, try to book as much ahead of time as possible, to require less cash while on vacation. If you can't book ahead, look up what the price will be, this will give you a good idea of how much you should take (always take extra just in case).
Other than that it depends on how much food and other items you are planning to purcahse while on your trip. Do you plan on purchasing smoothies, cookies, snacks, etc. that are not included? Are you planning on ordering room service? This will all require extra cash, or your credit card. For the tip though, cash is usually best. When you are on land, how many meals are you planning to get? I would say $100 should be plenty for two people to eat on while on land. Depending on where you eat and how many meals you plan on eating. You can always go back on the ship and eat if you like. Since you have already paid for your meals, this would be the most cost affective way to go.
Souveniers, jewelry, clothes, etc. is all something you have to consider as well. Obviously you will need lots of cash if you want jewelry. There is a lot of beautiful jewelry on the islands. Just be sure to stick with dealers that cruise line recommends. Going off the beaten path could end up in a bad situation. The cruise line will give you a good idea of good shops.
But to give you an idea I will usually take $2000 with me on a 7 night cruise. This is to be on the safe side. Most times you will be able to bring plenty back home with you, ocassionally you may not be so lucky, or really lucky depending on how you look at ;-) I also believe many islands have ATM machines if you need cash. Although I am not positive on this as I don't use them. But I would assume you might have pretty high fees for using them.
Also, another tip. The cruise staff works very hard and really does a great job. I honestly have never been on another vacation wear the staff caters to your every need as the staff of a cruise will do. Just keep in mind, these very hard working individuals are often away from their family for long periods of time and have little contact with them. Whenever I go on a cruise I will purchase several calling cards before I go (a few for each of the staff that I deal with through out the cruise, waiters, maids, etc.) then I will give each a handful. This is something that doesn't cost me a lot of money, but really means a the world to them. That way they don't have to spend their hard earned money and tips to call home.
I think this pretty much covers everything.
Have a great trip!