I can answer some questions for you ... San Diegan here.
In April in SoCal is the tail-end of the rainy season (March). Check the Weather Channel online for average temperatures, but I would say you are looking at the high 60s.
The drive from Monterey to Hearst Castle would be a tight squeeze time-wise. I would hazard to say it's a 3-4 hour drive each way. Monterey, however, has plenty of sights to see, according to my sister.
In terms of ports you mentioned, you may wish to look into a Mexican Riviera sailing on the Sapphire (which is leaving out of SF right now), or Carnival Pride out of Long Beach. I think you will get the warmer weather that way.
Hope that little bit helps.
__________________ Harry Martin
Moderator - Mexico and G/L message boards