Even though it wouldn't come close to making California an island, I'd like to point out that it's by geological luck that the sea doesn't still extent all the way up the Coachella Valley to Palm Sprints & Indio. Most of that entire valley is at or below sea level. The reason that the sea does not extend that far up is that it is held back by a natural dam - a large sand-deposit where the Colorado River empties into the Sea of Cortez. To put it crudely, the Colorado River carved out the Grand Canyon, carried it away, and built a natural dam with several hundred miles downstream, which now protects the Coachella Valley from the ocean.