I just came back from a Caribbean cruise, and I have porcelain skin that turns red if I think about the sun.
This is my wisdom, take it or leave it:
Buy sunscreen before you get on the boat--and not vanity sunscreen, something around 35-45. Apply it BEFORE you get out in the sun so you don't have to think about it once you're on the islands (put it on with your bathing suit--or better, have your husband apply it for you.) Apply it every 4-6 hours, more frequently if you're swimming or doing sports that involve sweat. Don't forget those weird places you'd never think you'd burn--like your ears or your calves. Your ears really hurt when they peel, okay? Don't forget them. And don't forget your back--especially if you're going to be snorkeling on those lovely islas. If your hubby won't lube you up, get one of those spray on sunscreens.
Wear sunglasses, because if you can't see anything there's no point in being on the islands
Wear some kind of lip protectant (a lot of chapsticks have sunscreen, so do a lot of lipsticks) because, YES your lips can burn and that hurts, too.
Bring aloe and skin moisturizer, because even if you don't burn, the sun will dry out your skin, and that makes it even more vulnerable. Moisturize frequently and generously. The aloe is for when you forget your sunscreen
You will get a base tan from gentle frequent short term exposure to the sun; if you're outside in the sun from 4-6 every day, you're doing a much better tanning job than if you bake yourself from 12-1.
You can always go back to the boat if you get too burnt to get a coverup shirt/hat/more sunscreen.
And, just think: if you get really burnt on the islands, you're going to be in way too much pain to enjoy the rest of your trip and that would totally totally suck.