News stories mention that these normally live in tropical and subtropical waters. However, the ocean temperatures in the Santa Monica Bay have been warm, even by our seasonal standards. Click on the Navy's global sea surface temperature (SST) analysis to enlarge the area off the California coast.
For reference, compare the climatological SST for this time of year on the same color scale. The Santa Monica Bay is experiencing offshore SSTs normally found halfway down Baja California (near the latitude of Bahia Tortuga).