Although San Diego is probably my favorite larger U. S. city, the month of March sees very changeable weather. It is usually windy and rainy more frequently than other months of the year. Highs in the day will probably reach near 60 to occasionally 65 degrees F, unless a Santa Ana wind is blowing off the interior deserts. I'd deffinitely take a good warm windbreaker and a fold up umbrella.
Be sure to have a margarita at Hamm's Lighthouse, just across the exit from Lindbergh Field on Harbor Island! Lots of things to do, so you'll enjoy it at any rate... Welcome to the U.S., by the way!