Sounds like a great trip. I love Shi Shi beach and the area around there. I do not get up there as much as I would like. It's amazing the amount of trash that washes up... sad really.
My in-laws have a place right on the beach at Pacific Beach. Every time I go stay it seems like I spend most of the time picking up trash as I walk the beach. Some days I feel I'd rather just sit inside and read as it's a bit depressing to see such wonderful areas littered with trash.
Kudos for making the long drive and hauling out such a mountain of litter.