San Diego is an ocean front city, so there are many places to walk by the ocean. Shoreline Park on Shelter Island is one of my favorites because you can see the downtown skyline view while enjoying a stroll beside the water.
The park is a mile long and basically a sidewalk between the ocean and a wide strip of grass. It's great for jogging, running or biking. There are picnic tables if you want to just sit and eat by the water. This is a great park where you'll find mostly locals (except during the boat show) and you can just bring a blanket or chair and lounge around. There's grass along most of the sidewalk and some benches in a couple areas.
You can watch the San Diego Yacht and Boat Show in July from here.