I've never been to the D.R. but I have been out on the big water a few times.
What I would say to that is YOLO. I've had very poor days on the ocean, some OK days, and a couple that were a damn tiring experience with reeling in fish. One thing they have all had in common is that your guides and their first mates will do all the work, that is they will bait the hooks, cast/let it out, and set the hook. All you have to do is reel. That last part is kinda disheartening where they grab the rod and make sure the fish is hooked before they hand it off to you. To me that is the best part of fishing!
Do some internet legwork on potential guide services and what is typically biting during your visit. If you get fixated on a given species you can really miss out on some fun. For example, this past June I went with my cousin to Fort Lauderdale for a chance at some large grouper. We didn't catch a single damn grouper, but I had two awesome battles with sandbar sharks in the 7 ft plus range (about as big as they get), a bunch of bonita which fought like hell on light tackle, a barracuda, and some king mackerel. It was a great day on the water despite no grouper or tuna.
Also, pay attention to what your guide service recommends for taking with you on the day of your trip. Some supply water, beer, etc and some don't. Shoes should be white soled so as to not scuff up the typical white fiberglass interior of offshore boats, sunscreen, and snacks.
Non-fishing people also often enjoy the day on the water as there may be turtles, dolphins, and ???? to see.