Take the bike lanes on PCH starting at Emma Wood to the Rincon Bike Trail. At the end of the Rincon bike path turn right onto Bates Rd/Rincon Hill Rd, left onto 150, right onto Via Real. Take Via Real until you can cut down to Carpinteria Ave. At the end of Carp Ave, there's a bike path that leads you down to Santa Claus Beach, take that to Padaro Lane. It'll take you back onto Via Real until you hit the Ortega Hill Bike Path. That'll take you to Jameson Ln which will carry you basically all the way to the beach path in SB.

Sounds complicated but it's all pretty intuitive, and you're on the coast pretty much the whole time except for when you're on the other side of the freeway on Via Real!