From Sosua to Baharona it is about one day driving only! I believe there are plenty of nice places arround Sosua to try and not having to go through the whole country! I have no personal diving expierience here and this board has never provided much to my knowledge, but you may find interesting places in the bay of Samana, all along the north coast or drive up to Monte Cristi and have a look arround over there! I have no idea, but there could be the possibility to rent a boat and go for the silverbanks, this is offshore but very interesting corals and reefs to look at! And it belongs as far as I know to the DR! They have found a couple old ships over there, but I assume you know about it (Nuestra se?ora de la concepcion etc.)!
Have a great time!
Henry