My friend who goes all the time said to take a Listerine bottle (totally cleaned out and sterilized) and replace it with scotch (same color). And regular water bottle with Vodka. We did that and got away with it - but a can of club soda was $5.
All in all, we spent as much as the cruise itself onboard. They make it very easy for you - no cash - just a Norwegian room card. And for $20/pp you can upgrade to the "special" restaurants where the service, food and atmosphere is better. Sounds fairly cheap but ended up doing it every night which added up.
Our favorite excursions were zip lining (St Lucia), pub crawling (Barbados), and kayaking off the coast of Tortola near Richard Branson's island. Bet they have all that going on in the Bahamas too.
One thing we learned about excursions is that the cruise line totally jacks up the price. When you dock, you will see TONS of local cabbies. They are begging for business and we had one take us all over the island we had stopped at for like $20. It was great because he showed us the coolest sights and stopped and we had a drink at his friend's pub then went to where he said the best food was. so less commercialized than getting on a bus with 100 tourists.
I never ever wanted to go on a cruise and we were talked into it. I am extremely prone to sea sickness, so I stocked up on meds - but never took a thing. And, after all I had said before about cruises. I LOVED IT and didn't want to get off. Sitting on the balcony in the morning with coffee, watching the ocean is like heaven
hope you have a great time.