New here as well - but did dive and snorkel on Big Island Sept. 2013.
Couple tips I can offer:
Honaunau Bay (aka "Two-steps") for calm, easily accessed waters and good visibilty - lots of fish
Kona night Manta snorkel trips - No freediving permitted - you have to stay at the surface and cling to a raft with many other people, most of whom have no experience nor understanding nor respect for the sealife. Very disappointing and super crowded.