San Diego is home to dozens of surf filled beaches along the Pacific Ocean, as well as dozens of calmer bay beaches. Shells wash up on every beach, but as the locals know, some areas of San Diego are much better than others for finding intact shells that can be used for collections or as part of a hobby or project.
The southernmost beaches in San Diego are Imperial Beach and Silver Strand State Beach. Imperial Beach is more for those just sightseeing and walking the beach, while the Silver Strand is one of the best beaches in San Diego to find various types of shells that are intact and easy to get. The Silver Strand runs from Coronado Island down to Imperial Beach, and you can spend many hours walking the shoreline and filling bags with beautiful and interesting shells.
Many tourists visit Coronado Island, but they usually just like driving over the bridge or looking at the famous Hotel Del Coronado. Not many visitors actually visit the beaches, which makes them prime shell-hunting territory. The southern area of Coronado has the largest beaches where you can usually fill up a bucket with shells within just a couple hours.
The three main central San Diego beaches are Ocean Beach, Mission Beach and Pacific Beach. These three beaches are heavily populated by tourists and locals alike, and the odds of finding large shells there are pretty slim. If you take the time to stroll along the beach to the smaller, less populated beach and cove areas, or stand out on the sandbars you can still find some amazing shells before they wash up and get discovered by others.
The two northern beaches where you are likely to find large shells that haven't suffered much abuse from erosion and tourists are in La Jolla and near the Scripps Institute of Oceanography. La Jolla has quite a few areas with beach access, and if you are a skilled snorkeler, the La Jolla Cove area has a wealth of shells 10-15 feet deep. Scripps has a pier and offers access throughout the year to their coastline as long as you aren't swimming or disturbing sea life.
The summer isn't particularly good for shell hunting due to the sheer amount of traffic along the beaches. The best time of day for shell hunting is early in the morning around sunrise when the shells from the previous night have washed ashore. You'll see many others competing to get the largest shells. If you know any locals, ask them about hidden beaches and coves that most people don't know about. You can usually find a large bounty at these local spots.