I just returned from San Diego today. We stayed one day in the Coronado Island Marriott and 3 days at the Marina Marriott. The Coronado Marriott is on the bay side and really isn't a beach if you are a real beach lover.
We stayed in one of the Villas. It was a long way from everywhere and ok, but nothing spectacular. The pool was ok as well. The beach is on the other side of the island at the Hotel Del Coronado (not a Marriott). We moved to the Marina because of additional family members joining us and couldn't afford the points for rooms on Coronado Island. That was a lucky break for us because the Marina Marriott is vastly superior to the Marriott on Coronado Island.
In my opinion, Marriot doesn't have a great beach hotel in the San Diego area. We loved the Marina and drove to Mission Beach (15 min) for all the surf, sun and fun we could handle. By the way, there is a no alcohol law on the beaches in Mission Beach/Bay area.
Have fun and let me know if you end up agreeing with my experience.