Due to the geography of the Monterey Peninsula, there can be huge differences in conditions at local shore dive sites. The prevailing wind and swells usually come from the northwest, although that can vary. Some popular shore dive sites like North Monastery Beach, Butterfly House, and the aforementioned Fanshell Beach are exposed to those prevailing swells and thus can only safely be dived when conditions are really flat, or perhaps when the swells have shifted to come from a more southerly direction. All of those sites also feature large submerged rocks right near the beach which can present a serious hazard in rough conditions.
Some other shore dive sites like Breakwater (San Carlos Beach) and Point Lobos State Natural Reserve (Whaler's Cove) are much more protected. So, experienced divers can usually get in and out safely there even in rougher weather conditions as long as they're careful. But even at those sites we sometimes have to pass on diving.