Ambon is the smaller island to its larger northern neighbor Seram. Ambon is beautiful, lush with tropical rainforest and quite mountainous. The diving can be excellent with 20 meter visibility to terrible, depending on currents, active runoff and dive site locations. I have only visited once and we did six dives, mostly within Ambon Bay. Ambon is known for great muck diving and associated unusual critters but I found some of the classical sites to be fantastic with submarine archways, hot springs (cooking eggs) and oh-the tame fresh water eels at a local kampung! The primary dive season is September to December and March to April.
Recommended dive operators
Dive into Ambon: http://diveintoambon.com