I was in the Cooks last Sept/Oct - one week on Raratonga and a second on Aitutaki. Wonderful place!
The corals are somewhat decimated by Crown of Thorns and bleaching but it's still great diving. Some persistent winds while I was on Raro limited us to the lee side and even nixed a planned shallow dive in the Lagoon for picture taking. But we could always find a place to go out and plenty to see. The high point was a close encounter with a mother humpback and calf at about 50' - awesome. Graham and Christina at Pacific Divers go out of their way to accommodate.
On Aitutaki, there's only one dive operation but it's a good one. The most beautiful lagoon in the world and some great rock formations outside. Saw lots of giant clams, Napoleon wrasse as well as the usual reef life.
Best of all, the Cooks above water are quite unspoiled - very laid back and relaxed...my husband and I loved 2 weeks without TV or telephone. Also, we were quite amazed at the quality and variety of dining - especially on Raro....and don't miss the 'Island Nights' buffet and dance show - especially on Aitutaki!
..and it's all very inexpensive when you convert NZ $$ to US!
I have heard there's been a problem with dengue fever of late, but hopefully they're getting that resolved because we're hoping to return in Nov/Dec this year.