Bunaken is a whole year desitination.
Best months are May to September (the dry season). Wet season from November to February. It has a little affect to the visibility.
Peak season July and August which is higher priced and fully booked.
Dolphin and whale watching is offered in July and August.
Bunaken is famous for its underwater wild life. You are able to spot more fish species during one dive than living in the Caribbean Sea incl. the famous mandarin fish. It has five species of sea turtles and even white- and black tip reef sharks.