I just got back from doing a Caribbean cruise. I've done 8 cruises, and have been to all those places except St Kitts. If you're doing excursions with the ship line, it will be expensive. I've only done 1 or 2 excursions with the line. A lot depends on what you want to do. Go over to Tripadvisor.com and search out the different activities on all the stops. It will give you a good idea what is available, and customer revues. A lot of times you'll be able to book the same identical cruise in port and often with the same vendor the cruise line uses. It will be cheaper.
You'll have to take a cab to town if you want to see anything in St Thomas. It has a large shopping district, if you're into that. Also have an old fort that may be worth a visit, as it's just been restored. There's also a place to sample rum. We have always taken a cab tour of the island. On one tour, they dropped us off at Coral World and gave discount cards. They came back and picked us up and had us back in plenty of time. You pay your cab fare to these guys when they get you back to the dock. It gives them incentive to be there on time. Never had a problem.
On Barbados, we also took a cab ride. Found them right off the dock. They took us all around to the top spots on the island. You also get to talk to the local people and get a better feel for the area. Cost was $60 for the ride and lasted a couple of hours.
St Lucia is a bigger island. It is one that I did take an excursion to the rain forest with the cruise line. It was fairly expensive, but it was worth the money. They do make stops that have people constantly trying to sell you trinkets. That's how they make their living. Make sure you see the pitons when you go past.
At St Marteen, we just walked around the town on the Dutch side. Beer is cheap, and you can get a lounge chair for a few bucks on the beach. If you want to relax, its a good way to spend the day. The town area is full a hucksters. They are in your face. We ended up making the mistake of taking some kind of "free" ticket for a chance to win something. Ended up being a time share, and we spent a lot of time trying to get out of there. If you feel real adventurous, go to Orient beach.
Most cruise lines want you to spend money on drinks. That's where they make their profit. The cheapest bottle of wine you will find is about $25. An average mixed drink is $7. A beer is $5 and a soft drink is $3.50. They'll throw a 15% gratuity on top of that. We take water bottles and fill them up in Lido. Most lines are now pushing drink cards. If you get one, everyone in your room needs to buy one. Breakeven is at 6 mixed drinks a day. It takes more of the cheaper drinks. I never thought they had that much value, as you won't be using the drink cards when in port.
March is a great time in the islands. Not too hot, and nothing more than showers, if at all. Seas are fairly calm.
