I've been diving with both Extended Horizons http://www.scubadivemaui.com/ and Mike Severns http://www.mikesevernsdiving.com/index.html in Maui and found them both to be excellent.

I've given this reference before: Scubaboard (the Flyertalk of scuba) has a very active Hawaii board http://www.scubaboard.com/forums/hawaii-ohana/