Easy! (1) You don’t need an efoil board, you can use ANY foil board. (2) You can mount torpedo at top or bottom of mast so you could use as efoil or wave assist.
To preserve board integrity, kind of PWR foil solution…
We could imagine a cylinder full of batteries close to the board and balanced between the rider’s legs (kind of submarine looking) and a “traditional” efoil motor close to the foil fuselage.
What if the foil fuselage is discarded and the torpedo is used as a fuselage itself, with wings and mast connected to it. This way the drag and weight of the original foil fuselage will be discarded too.