In northern California, it's often cool enough to require covering most of your skin. A person surfing at Linda Mar is almost certainly wearing a wetsuit and gloves. This covers almost all of the body and drastically reduces vitamin D production.
To reliably get vitamin D, take a supplement. Exercising outside is more fun than a gym, but it won't make you any healthier than doing the same thing indoors.
To reliably get vitamin D, take a supplement. Exercising outside is more fun than a gym, but it won't make you any healthier than doing the same thing indoors.