Depending on what you are looking for—a blue-water golden fish or deep-water exotic species—you can choose snorkelling or scuba diving in the Philippines. The dry season is preferable, as the water remains clear as glass and smooth. The problem is climate differs across the islands—owing to four different climate types there's no one and the same dry season for the Philippines overall. November through to April are known as dry months roughly for half of the Philippine Islands. So this period may be assumed as the most favorable for diving but in fact, diving is a year-round activity.
Among numerous recommended locations are the Tubbataha Reefs Natural Park, El Nido’s Bacuit Bay, Bicol, Bohol, Panay, and Mindanao or Mindoro.
If you're interested in wreck diving, there's nice spot around Coron, the seabed landscapes off Palawan and Subic Bay have largely changed after World War II.