Could it possibly be Satan's Triangle?
A USCG pilot and his winchman Haig (Doug McClure) answer an SOS call at sea and arrive at a derelict schooner called the Requoit. Haig lowers himself to the ship, where he finds three dead bodies along with one survivor, Eva (Kim Novak), cowering in the cabin. As the pilot attempts to retrieve Haig and Eva with a rescue basket, the line breaks, plunging the two into the ocean. After they swim back to the boat, the pilot informs Haig that he must return to base because his fuel is borderline. Eva and Haig spend the night on the boat, during which time she recounts the story of the storm that killed everyone else aboard. One man was hurled through a hatch; one is hanging from the ship's mast; another vanished before her eyes; and a fourth man is in an aft compartment, floating in the air. Eva attributes the deaths to supernatural causes, but Haig has a practical explanation for everything, including the man who appears to be floating in the air. In the morning, the pilot returns along ...
There's a swordfish death (the man seemingly floating). Ironically enough, it's a lot easier to find the complete movie online (although any individual link eventually gets shut down) than it is to find a trailer.
- Initial person picked up by the people on the yacht was a male floating in the ocean.
- Final survivor recovered after a rescue call.
- Deaths get explained.
- Impalement by swordfish
- Film was released in 1975
- Title mentions a triangle and Satan.
- It does end with the lone survivor back in the ocean, about to be picked up by another ship as he laughs sinisterly.
Parts that don't match:
- Survivor is apparently female (although later revealed to be the priest who was initially rescued and was thought dead, hanging from the mast).
- The Devil allusion happens much later, after the rescue, although Eva discusses her fear of the Devil being behind it all throughout the movie.
I'll admit that I found the movie mentioned in a Movies SE question.