Kauai in May, you are in for a great time!
We have made several trips to Kauai and love this Island and the diving is terrific! We always stay on the Poipu side at a nice condo. (
Kauai Vacation Rentals featuring Aikane Kauai Vacation House at Poipu Beach, Kauai tell Chris we sent you!) Not crowded at all and a very nice beach, right where the rare Orca beached itself! Poipu Beach is my idea of the perfect Hawaiian vacation spot.
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, Kauais 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