Cold shock reaches its maximum intensity between 50F and 60F, so you have ample opportunity to experience it on the West Coast. As a general rule, hypothermia takes 30+ minutes to occur (core temp <95F) even in freezing water. Faster for small, thin bodies, much longer for large, fat ones.
Lots of info & 19 Case Studies of close calls & fatalities.
Think about where your mouth is relative to the water surface (3-4"). If you become physically incapacitated by cold (helpless & unable to use arms & legs) while floating in the water, the threat you face is drowning. Hypothermia is a tertiary threat. Without thermal protection, you will most likely drown before becomming hypothermic.