Actually just got back from DR last Saturday.. Stayed at the Grand Oasis, which is a great hotel. Beach was 5 star, but the fishable water was pretty nil. Purchased a 3 piece st-croix specifically for the trip, thinking I might find some fishable water. Apparently, it is forbidden to develop beaches, so I thought I'd find some great flats. I did find some flats, but anything you'll see inside the breakwall, is tiny.
Funny thing, I did see a local catch a moray eel and a small ray from one of the docs, but everything else was basically bait...
My advice would be to book a charter for deep sea fishing. BOOK THROUGH THE HOTEL. I made the mistake of booking through a local outfitter, and needless to say, we're lucky to have escaped with our lives.
Send me a PM if you require any more info.
Have a great trip either way!
cheers
HD