Mark Twain famously said, the coldest winter I ever spent was Summer in San Francisco.
Our typical summer weather pattern on the west Coast of California is Fog in the morning, burning off to warm in the afternoon. Then cooler in the evenings and the Fog rolls in again.
On a California cruise, I would definitely take a sweater or even a jacket. Remember, a ship traveling at over 20 knots creates a wind chill that even in relatively warm temperatures, will make one feel colder that it actually is.