I speak as a cruiser, looking for something interesting to do in the six hours I have in your port. The cruise ship excursions are usually well over $100 pp for a full-day excursion, on a bus with people who are using walkers. So people on Cruise Critic band together, looking for a local provider who can give them a good day in a small van. Say, you can put 10 people in the van at $75 pp for a six-hour outing. Give them a cooler of beer and pop, and take them to some nice places. Make sure the van is not a pig-sty, and take them to see a waterfall, a mountain village, a banana plantation, and then a beach for a swim. They are gonna be happy as long as you get them back to the ship on time. They will also tip the guide and the driver cash.
If you give 4 people a personalized taxi ride for $200 for three hours, they will be happy if the car is clean and the driver takes them to some pretty places, talks fair English, and gives them some explanation. They will also cash tip at the end.
HOWEVER, these people have to be able to book this online. They plan their trips before they board the ship. If they can't find an alternative online, they will book the over-priced excursions the ship offers.
This is the opportunity. I am sure Dominicans can rise to it.