September is still warm typically in Southern California -this coming from a local. As with any place,your mileage may vary, but often it is warm until Halloweenish (October can be cold, but the last few years has been warmer). California usually gets cool enough for a jacket in the evenings. The first day or two of the Mexican Riviera can be cold (but more from November to March/April), but it is usually sunny/warm once you are in Mexico. Also, the water will be cold, or cool, never warm.