I took my Open Water cert with Jack's Diving Locker in Kona a few years ago now, and still have very happy memories of the experience. The instructors and customer service were top notch and a lot of the staff from then are still there today. The night dives to see the manta rays still rank among the most impressive dive experiences I've ever had.

Compared to places like Thailand and Egypt, diving in Kona isn't cheap, but my experience was of a very well run operation and I still think it was money well spent. Just typing this makes me want to go back!