A ship captain makes his living catching fish, but his livelihood is allegedly threatened when the "killers of the sea" move into the area, devouring his fish. Such killers include sharks and "whales" (actually dolphins, which are whales in a technical sense). What does the good captain do? He takes on the killers with his mighty wrestling skills.
This film was falsely marketed to me in a box set of classic horror films. I would have to make some amazing stretch to call this a horror film... it's not. Seemingly, this is a documentary... the captain appears to have been a real person, though my brief search was not able to turn up any evidence of this beyond the fact he is credited as himself.
Other than the misleading aspect, I actually enjoyed this film. Not as a horror film, and maybe not as a documentary, but as a time piece. Apparently at one point it was perfectly acceptable for a man to wrestle a dolphin to death for the sake of increasing his fish harvest. No "dolphin safe" nets here. And also, I guess you could get away with dubbing a clearly racist voice over the black shipmate to make him sound even more foolish and bumbling than he already came across.
Other reviewers have pointed out the obvious unintentional meaning of the title: killers of the sea. The fishermen are the ones indiscriminately clearing the sea of fish and personally killing the creatures who get in their way. I won't say any more for fear of sounding political, but it's an interesting time piece, as I said. Check it out if you have a spare hour.