Just thinking outside the box here.
I have just finished building an electric surfboard (not a foiling board) (video of 2nd run here: https://www.youtube.com/watch?v=81WGsLw6IYQ ) and have absolutely no intention in building a foiling board as the place we use the board has lots of water plants making it impossible to foil (even the big boats have problems with them).
The jets (the board has 2) has unbelievable power. In the video the jest are running 1/2 power).
So… Me thinking… Did anybody ever tried using a water jet instead of a propeller as propulsion?
I am sure that a jet and motor will not be much bigger than what is being used now, and it could easily be made into a hydrodynamic package.
Also a jet (pump) efficiencies is around 90% or more than a conventional propeller (60%-72%)