Check out :
Luperon Yacht & Boat Sales ~ Dominican Republic ~ Page One ~ Punta Alma Luperon
There are charter fishing boats working out of PuertoPlata, Sosua, Luperon, Boca Chica, and probably on the east coast by Punta Cana/La Romana. I'd recommend staying in these towns and talking to the charter boat owners before seriously considering it. It's not any easy business. There are a lot of taxes, licenses, and regulations and a lot of competition. On the other hand if you are considering it you would probably have fun checking it out in person.