We usually go to Hawaii for a week each year during the winter, and planning to go to the Big Island in mid-February. So Cal water temps are still a little warmer than usual for this time of year; I had 61° last weekend, but others reported mid-60s at Casino Point. We usually plan for low 70s in Hawaii, which isn't bad for one dive, but for two or more dives a day on multiple days you definitely start getting more and more chilled as the days go on. We usually dive 5mm full suits with hooded vests underneath, and gloves. If we're down for over an hour most dives and not kicking much, I'm definitely cold part way through the second dive, and my wife getting cold is usually the limiting factor for bottom time
You didn't say what kind of diving you do here. One shore dive with a hike while fully geared up to the water and a reasonable surface swim keeps you a lot warmer than riding out on a dive boat first thing in the morning (possibly in the rain), and then getting wind blown between dives. I found a big part of getting cold with diving in Hawaii is how much you can warm up between dives. The food most of the dive boats serve in Hawaii between dives always seems to make me colder on the second dive
Personally, I'm planning to take a 5mm full suit and hooded vest for diving, with a long sleeve Lavacore top to wear underneath if I start getting cold