I wonder if lifeboats couldn't carry a little device that sounds (or smells) like a killer whale. Maybe sharks are never really that much of a problem.
In the case of the Farallon Islands it might be that once Great Whites can smell that the orcas have eaten one of their own they know it's time to escape.
I've seen at least one documentary where smaller sharks disperse when they hear the calls of orcas. Seems like it could at least be somewhat effective in areas where orcas regularly prey on sharks.