My diving has pretty much been exclusively Monterey and Honolulu.
I don't know of any "cold water certification" but it's definitely much more challenging here than in Hawaii. You've got 60+ dives, but it's hard to give targeted advice without knowing where and what kind of dives these are, and what you general experience is.
In Hawaii, you'll typically be diving an aluminum tank with a shorty or 3mm wetsuit. The water is 75-80 degrees with little thermocline, and a bad vis day is 30ft, with 60-100ft viz not atypical. The reefs off Waikiki aren't typically chock-full of life, but there are some interesting and easy wrecks just a few minutes from harbor, and you're very likely to get up close and personal with some big, friendly green sea turtles.
In Monterey, you'll need the thickets wetsuit you can find, typically a 7mm+ or farmer john style, or a drysuit. Even then you'll probably end your dives being cold. 30ft viz is a good day, and it can go south pretty easily (days with 5-10ft viz aren't all that uncommon). Water temperature is usually 48-53 degrees year-round. Most people typically dive steel tanks to help offset the buoyancy of the thicker, bulkier wetsuits. It's *not easy* to move around with all the extra insulation and gear you'll need, and you definitely want to buddy with someone familiar with the area/site. It's very true to say that if you can dive NorCal, Hawaii will be an absolute cakewalk. The best thing about Monterey is the incredible abundance of sea-life. A ten-minute surface swim from short gets you 30-60ft of depth with reefs full of fish, inverts, and kelp rising to the surface like underwater cathedral columns. We occasionally have juvenile seals follow us and play with us, bumping and trying to steal our fins. IMO it's really hard to beat, but it really depends on what you're into and what you want to see (and what you're willing to put up with).
September is great in Monterey. It's effectively summer, the kelp forests are still around, and it's probably a tad cooler than August. Same with Honolulu diving - you'll be dealing with marginally stronger swells on the leeward side than in the winter months, but the water will be be 5-10 degrees warmer than it'll be in 3-5 months.
