Diving in the DR is not that great compared to Bahamas, Cayman Islands, etc because the DR has mountains and rivers which cause:
1) Rains bring silt, mud, and dirt to shore lines and reefs, causing visibility issues.
2) Rivers bring bacteria (like feces) and other pollutants into the waters, killing reef and fish.
The South (Catalina Island, etc) and North-west (Punta Rucia) have less rain and less mountains, hence better visibility.
Note that Bahamas and Cayman Island do not have any mountains or rivers.