You can easily picnic right on Main Beach. It's the iconic Laguna Beach beach. But for restaurants on the ocean, most are pricier (no shock there). One place we really like is Splashes in the Surf and Sand hotel. It's a little more expensive but right on the beach. Fantastic views and very good food.
One other place that's casual is Beachcomber Cafe. Make reservations here, it's really popular. Even on off-peak times. But literally on the beach, great food, and a cool spot about 10 min north of Laguna Beach.