I've ridden cold water/air for 30 years and I've tried practically everything. Best thing I have found is a good 3/2 wetsuit, 3 to 5mm on your feet, a 1.5 mm rash guard, good hand warmers and summer gloves, and a float coat PFD. The float cost is the key piece of gear, they are amazing!
Everyone has their own favs, but I can tell you that this combo very works well in 30 degree water temps and I'm never cold while riding. Now someone come up with a good combo for post riding and hanging out on a 30 degree beach.....that's when I get cold!