Tides at Half Moon Bay
Half Moon Bay lies just north of Pescadero State Beach and Bean Hollow State Beach.
If you want to clamor around the rocks along the shore to check out the sea life, including the barnacles, you will want to go at low tide. Given that the tides vary by numerous feet along the Northern CA shore, the window of low tide tends to be pretty small.
This site provides excellent tide tables. Better yet, they offer iCal subscriptions to the various tide tables!