The water at Kona certainly isn't California cold but its not Bonaire warm either.
Should probably plan on a 5 mil suit there.
I dive Kona in a tee and trunks all year round. A thin suit, say 3mil is about all you'll ever need here ... but you will not overheat in a 5mil.
I realize the OP no longer has an issue, but I just had to address this, because I totally disagree. Having dived both, I would leap at the chance to dive the Channel Islands again, but I would only go back to Kona if I had a reason other than diving to be there. To me, the diving in the Hawaiian Islands just hasn't been that big a thrill. Other than the fact that it's warm and the water is generally clear, there hasn't been that much that has grabbed me.
I have to agree with Lynne, except for the Mantas, the Spinner Dolphins, the Humpbacks, the Marlin, and the Tiger Shark.
But that's all snorkeling, not scuba.
On the other hand, the Channel Islands are so packed with life and beauty that it's stunning. Floating through kelp forests with the sunlight slanting down, like sun rays through redwoods, except the "birds" are schools of blacksmiths and kelpfish. Rock walls covered in strawberry anemones and coralline algae, in vivid colors, and dotted with sponges and nudibranchs. Warmer sites full of pugnacious Garibaldis and schools of rockfish, and the occasional capering sea lion. I've done two three-day liveaboards there, and several single days of diving, and I have yet to tire of Southern California.
California diving is, to me, the most beautiful in the world. As I said in an
earlier post (with reference to Lobos, but the Channel Islands can be close): "This had become one of those spectacular central coast days, blue sky, bright sun and 60-foot plus visibility. On a day like this Blue Fin Cove is perhaps the most spectacular dive site in the world. Let the tourists have Palancar Reef, the wall on Cayman Brac, Rosh Muhammad and Heron Island, all that frantic motion and frenetic neon of the underwater Times Squares. Give me kelp, the kelp forests, subtle deep greens broken by shafts of light that look like a Sunday school painting. That's for me."
I do think the comments about not being accustomed to cold water diving are apropos, though. For me, the Channel Islands are dry suit country -- we had water in the mid 40's off San Miguel, and in the mid 50's everywhere else, and that was in June. We had two wetsuit divers on the last trip, and they had fun, but they had 25 minute dives, where most of the rest of us were enjoying an hour or more.
Dry suit would help, the least I'd use is a 7mil farmer john plus jacket (step in preferred).
... plan on 7mm wetsuit, hooded vest, gloves, and thick booties. In fact, I usually take 2 pairs so I can let one pair dry out on alternate days. ...
One pair of boots are fine, no need to dry them out, pour some warm water into them in the morning before you put them on and everything is wonderful.
Probably not enough time between now and 10/12 to find a dry suit that fits and get certified though. Not without paying $$$ for individual instruction.
I've rarely know drysuit courses to be anything but individual, many places throw them in for free to sweeten the deal.
Some posters may not have noticed that Kona is not available...?
There are alternatives to a liveaboard, especially in Kona. Stay at a nice hotel or a good B&B and dive with a local boat operator.
That said, I love the Channel Islands, but the diving there can be somewhat more challenging for a beginner.