Vehicle insurance in Baja and Baja Sur is optional. What could happen if you would get into accident without insurance, is another story.
I doubt that any US insurance will cover as far as Bay of LA. Not to mention dealing with accident and claim. When you buy Mexican insurance through those providers mentioned above - Lewis, Bajabound etc - you are dealing with Mexican insurance company. Phone reps speak English, though with heavy accent, and agent arrives at the scene of accident promptly, to avoid any complications with police.