Don't know about the south coast but fishing on the north out of Cabarete, Sosua, POP is pretty lousy! I have the same fishing bug as you but am sorely dismayed at the lack of response to my dangling things in the waters around here. Tried it all! Surf, beach, boat etc. there are fish but few and far between and probably the worst place in the world where I've ever fished!!
I get my buzz just fishing for small fry in our local river - at least that way I get to sit for hours alone and unstressed, but with no fish it's becoming a chore!
If you are serious about buying here, and about fishing, perhaps we could team up and do the deep water sport fishing thing? Local tour boats are very few, expensive and don't really go look for fish other than to take tourists out for the US$'s. A 4 hour round trip hardly gets you out into the dark blue waters, does it?
I was impressed with Samana area as it seems to have fishing potential but the only decent boat there was laid up or for sale(?) and we live too far away to make it a regular trip.
Perhaps we'll meet up if you're down this way and swap tales.
Walk softly, but carry a big fish!
Peter