I agree this scenario presents a quandary—with no obvious solution.

People have always swum in the oceans and always will, so I guess
it's a matter of continual vigilance by swimmers for any indicators
of likely shark activity.  When I was a kid, I virtually lived at the beach
and in the waters of Port Philip Bay in Melbourne, and there were
often shark warnings sounded.  A couple of the local radio stations had
light planes in the air circling the bay, and reported shark sightings
to the closest life saving club which then cleared the water of swimmers.