I went to the Cook Islands in September 2000 (during the Sydney Olympics) and spent the whole of the first week underwater - four dives a day for 6 days on Rarotonga, and then 2 dives for 2 days on Aitutaki.
I agree that Pacific Divers are the ones to go with - their service and knowledge was outstanding. And they give you mints after each diver so you can get rid of that terrible post-dive breath.
The very first dive I could hear this unusual bellowing sort of noise. Thinking it was my dive buddies doing funny things with their regs, I ignored it. It wasn't til we were climbing back on the boat that I was able able to see what the noise was coming from - a beautiful whale breached about 15 metres away from us! He'd been singing to us underwater!!!!! :luxhello:
Saw lots of nudibranchs, reef sharks, octopus, moray eels, eagle rays, great variety of fish, turtles, etc. There was a Crown of Thorns problem when I was there, but I believe they have had several treatments to inject them to try to deplete the population.
I definitely recommend the Cook Islands for a relaxing dive holiday - the dive sites are never more than 10 minutes away by boat and the locals are so friendly! (and I think the NZ dollar is the only one weaker than the Aussie one!)