There's an awful lot of sea and a very small number of propellers. Plus if the body is floating, it would not be anywhere near anything but the smallest outboard prop. Also, if you look at the shape of a rotating prop, it could clip you, or mangle you a bit, but for most propellers the leading edge pushes you away so won't act as a cutting device unless you push against it or it's at high speed. I would think it very unlikely a propeller would do what was claimed.