I appreciate Cecil’s noting that sharks have very keen hearing. In 1946 my father was stationed on Midway Island. He learned that when the garbage scow went out and began dumping that sharks appeared quickly. Flying around the island, he often spotted a group of sharks, sometimes a few miles from the scow, but noticed that when the began to dump garbage, the sharks would immediately turn and head for dinner!
It would seem that sharks would at least be aware of and alerted, if not attracted, to swimmers thrashing in the water.