My wife and I love cruising. We are going on our 18th in January.
Most of the cruise excursions you can get less expensively on shore. The one exception was the sugar train ride. Its cheaper when purchased from the ship. It was very interesting if you like trains. It gives you a trip around the island and they narrated while you are traveling.
Get your souvineers on shore away from the dock. Never take the shore vendors first price. If you do they will laugh at you behind your back and call you stupid. Its their culture to negotiate prices. So negotiate. I usually offer half their price. I usually pay 60% to 75% of the origional price.
We always ask our fellow travelers where they are going to go on the next island while at the dinner table. If they are wanting to go to the same places we want to go we teamed up with them and then went ashore together and negoiated a better price from the tour operators on shore.
If they have the "Chocolate melting cake" at dinner time, be sure to get that. (If you like chocolate.)
Go to the next island talk whenever they do one each evening. Sign up for the ship's tour if they offer one if you want to see what goes on behind the scenes. They won't take you to the engine room, but they will take you to other interesting sights the passengers never get to see.
Meals: They offer all kinds of food. This is the time to try something new. You can also have more than one item on your menu. If they offer 6 appetizers and you can't decide between two of them, order both. Go look at the menus posted on the door of the dining room. If you like what is being served, eat in the dining room. If you don't look to see what is being offered on the buffet. Then chose where you eat.
If you are a juice and soft drink person. (We don't do alcohol.) Wait until you are out of port and buy their "all the juice and soft drinks you want" program. It costs about $50, but then you never pay for soft drinks or juice at the bars through out the cruise. (Tips are included) Its a real good deal.
Sea sickness: If you can read a book while someone is driving on a curvy road you won't need anything. The big cruise ships have gyroscopic stabilizers weighing tons. Cruise ships are so stable now that they have miniture golf courses and bowling alleys on board. My wife cannot read a book while I'm driving on mountain roads, but she can read on the freeway. She has been sea sick only twice on 17 one week cruises and that was on one ship that we sailed on right after a huirricane went through. The sea was really rough.
You will have a wonderful time. If you are sun sensitive, bring a hat and sun block. If you normally use spf 30, bring spf 50. Have it packed in your suit case. If you have it in your carryon bags it will be taken from you. Sunscreen, batteries (camera, etc), and other common items are much cheaper here than on the ship or on the islands.
I brought an empty quart ocean spray juice bottle with a wide mouth with me. It is very useful in taking drinking water from the ship to the shore. They will charge you $5 for ship supplied water to take with you.
If you have other questions, e-mail me. Good luck to you and yours.