Quivira Basin is the cove just to the right of #7. You can see the name ‘Quivira Basin’ in cyan color. It’s an inch or so to the left of Sea World.

All picnic areas in Mission Bay are ‘first come first serve.’ There is no way to reserve an area. As for a good time to go, ask the kayak people. They should be able to give you some idea.

If you absolutely need to ‘reserve’ a specific picnic area and cost is not a problem, these people can help: http://www.picnicpeoplesandiego.com/
They ‘squat’ on tables in the wee hours of the morning. Several years ago, I hired them for a corporate event. They got me the perfect location with no problems.