could have had a underwater Sea Doo propulsion device.
It's true, but I have a lot of underwater experience below, from, between and to rubber boats, helos, subs, mini-subs etc. The degree of complexity of having a waiting diver or divers puts this "frogman assassin" theory into the realm of Hollywood, where we see Tom Cruise etc parachute into the water and swim to a waiting submarine all the time like it's some simple task. IT IS NOT!!!!!
If you want to kill Fuddy, you need the pilot #1 to ditch the plane in a safe, calm, flat area. He MUST be the critical part of the plan for it to work. (Hitting an open ocean swell the wrong way would be like flying into a hill.)
If you need an assassin besides the pilot, then he is going to be on the plane, waiting to cause Fuddy's heart attack with an aerosol spray, or by drowning her.