The sailboat completed the circuit to ground (don't know the voltage of the wires, but could easily be 440 volts--a very significant level). Just like a bird sitting on a wire does not get electrocuted. As long as the people do not form a path to ground--that is place one hand on an "energized" part of the boat and some other part of the body in the water or to another path to ground, they are relitatively safe. But there are all sorts of risks and scenerios where they would not be--if the path of resistance thru their body was less than down the aluminum mast and to the keel or where ever the path to ground was.
The PWC is isolated, and not part of the circuit. A person jumping into the water is probably safe--but If I was jumping (which I would) I would land a good distance from the boat.
I don't think that the PWC is connected to the boat--but a nylon on dracon line is a good insullator, the water may be somewhat conductive. But again it is at the ground potential, as long as it is in the water.