I went to Alaska on Royal Caribbean's Vision of the Seas 2 years ago. We had a marvelous time. They will issue you a credit card for all on-board payments. When we first got on board they were offering people lovely alcoholic drinks to welcome them. We thought they were free. They weren't but we sure didn't get caught again.
If you are non-drinkers you should not have a problem on board. All non-alcholoc drinks AT MEAL TIMES are free. If you go to the buffet dining room all drinks will be free. But you will pay for the drinks in your room and for any you get at the pool bar. Basically if a meal is involved don't expect to pay. Their lemonade is great so enjoy. It is free all the time in the Buffet dining room. I'm a cola drinker so had my fill of that at meal times.
As for the water - really no need to buy any at the ports. The RC has its own desalinization plant on board. The water is fresh and tastes great. We didn't get sick on it once. You will not be drinking "island" water.
Our RC cruise was fabulous. We woke every morning at 5 and went up on deck to watch us pull into a new pport. Then we went for breakfast - awesome food. If you don't like it - don't feel back. Your waiter will trip over himself to get you something else. If you like eggs make sure you ask for them well done they tend to make them a tad runny (unless you like them that way).
Everything else we ate was fantastic.
If you take a tour always arrange it through the ship. If the tour is late they will wait for the tour to get back. If you are on your own and they ship leaves you are out of luck and will have to find your own way to the next port.
During our week on-board RC (that we were on) had only one midnight buffet. There is a fast food diner for midnight snacks or room service $$$. We stuck to eating only at mealtimes - did a lot of walking around the ship and didn't put on any weight - and believe me we ate at mealtimes - a lot.
Enjoy the trip - it is well worth it. BTW when we got off the ship our credit card only had charges for 2 portraits that we had taken. They take pictures everywhere and you can go to the gallery later and find yours. If you want them ($25US each) then buy them If not drop them off in the bin provided.
Make sure you take at least one dressy outfit for the special nights and a suit or jacket for men. Unless you don't want to dress up - you can still go to the buffet on special nights where casual is the rule
If I think of anything else I will be back