It's a mixed bag. One of my favorite places to fish is in the wells out at the end of the Santa Cruz Wharf. I've caught many fish there including some big perch, kelp greenling and even a lingcod. Almost always the cross beams down near the water are covered with sea lions. Ditto the Monterey Coast Guard Pier that often sees sea lions covering many of the the shoreline rocks while fishing remains good just a few feet away. They rarely bother the lines. I don't think I've ever left a pier due to the sea lions although, as said, I've seen them fight anglers for fish on the lines at Balboa, Newport, the Cabrillo Mole and other spots.