In the 1977 Version of the film

RIGHT at the start, the camera pans down to the boat with the 3 men in it, 2 wake up to find the third dead, so they go about moving him to the edge of the boat to throw him out.

You can see the "dead" man helping them, by gripping the side of the boat to pull himself over, then letting go and playing dead again. He is then tossed over the side and sinks