skark!

ok i dont have any pics but my cousin and i were spearing in about 6 feet of water about 100 yards off of the beach in oceanside california. i look at him just in time to see his eyes go big, he said shark, now the vis was only about 8-9 feet so i knew it was close but couldnt be that big or i had seen it. so i didnt worry to much. we get back to shore and he described it, he said 4-5 feet long with black tipped dorsal and tail fin. instantly i think of black tipped reef sharks but i think im to far north, im about 70 miles north of baja. any ideas what it could have been?