You're timing isn't the greatest.
The Mona Passage, between the DR & it's neighbour Puerto Rico has some excellent Marlin fishing but your timing is wrong.
Winter brings colder water and the bait changes, the Marlin leave for warmer waters and that's that.
There are now some decent boats at the Marina in Punta Cana-although it must be said that the best boats come over from
PR to fish the season as it makes the daily run the grounds that much shorter.
So in the end you might hook into something-Dorado/small Tuna/Sail-anything you do catch will be killed unless you VOICEFEROUSLY object/offer a bribe for it's life before hand/preferably both.
The DR isn't on anyone's list of decent Sportfishing destinations for a good reason-crews are generally poorly trained/paid amateurs who are used to doing Booze Criuses/fishing equipment was old when Moses built the Ark/ boats are scows of the oldest/slowest type.
Vancouver Island it ain't (!)