As said above, it depends what you expect. If you want to see BIG stuff, this is not the right place. Even though I saw last month couple whales (whale season ending now), couple months ago a hammer head and tiger shark. But there is no guarantee you will see anything like this.
Most of the divers that tried diving at different parts of Dominican Coast will tell you that the best diving is on the south side (the Caribbean coast). More choices of dive sites, day trips with diving, corals and marine life not as damaged as elsewhere.
Fernie, I haven't heard about diving around Samana much, the area is famous for whale watching.
lynnemari - I would suggest if you have a choice, to go to La Romana area (Bayahibe).