He sees sharks swimming very close to the shoreline, he knows there are moray eels living in the crevices, and the birds have their nesting grounds on the beach nearby.
German submarines may detect they are on the island has nothing to do with swimming or water.
Answer:
They think that Betty has been bewitched so if she really was she would not be able to survive hearing the Lord's name. More likely the reason was that she heard everything about the affair between John Proctor and Abby.
