Diving:
All of the dive operations we have used have been great.
But here is my abbreviated list:
For shore diving, guides and gear with outstanding Dive masters, Aquatics Adventures does a first class job.
Aquatics Adventures
To Dive Niihau Island, KauaiÃÔ premiere dive location (subject to weather closures) and not to be missed. SeaSports Divers did a good job with competent crew and Dive masters.
Seasports Divers, Poipu
Local boat diving, I really liked Kauai Aquatic Scuba. Small boat and very nice people, good crew and Captain. Easy to work with.
Kauai Aquatics Scuba/Mana Divers
FathomFive also receive good reviews by others here but alas, I've not dived with them yet. There is also Bubbles Below, I used them a few years ago and things did not go to well but recent reviews by others have been very good!
Below is a link to a complete report of my last visit.
Kauai Dive Report - Links to Video/Pictures