Hehe with me totally the other way around. I would much prefer the get a ready made one, 2-3-4.000 $, order one and be done with it. The "sur-ron" of e-watercraft if you will. But I don't think that will happen any day soon. I am for sure not sinking north of 10 000$ like the ready one ships for today. Too rich for me for just a toy for those sunny days. Not only for sunny days, but you need to have time off to be able to use it so probably more of a holiday or weekend toy for the short nordic summer. Diy is the only way at this time at least.
For another project I was in contact with the company that makes the motors that matt uses for the davinci drives, those astro motors. And those guys could actually make custom variations of any of their motors. Like make em wider, two on the same axle etc. But the price sky rocketed for custom work, and those are not really cheap to begin with. The one they recommended for high power applications was a 4xxx motor. Iirc correct that was about a 2000 $ motor stock - then add custom work. And where do you find controllers capable of handling 25-35 kw of high rpm motor? Possible the nuclear big daddy controller? Will it handle the e-rpm?
The thing is, if there ain't any new light weight, compact high power motors I don't know about yet, an electric water craft with enough torque to make if fun and hi enough top speed to make you wanna ride it it's gonna be a lot of money just for the parts alone. Other then that it would an amazing projects. I have vast experience working with composite material, carbon fibers and fiberglass so that part of the project would be very fun. But motor, controller, and battery pack will eat a lot a cash. Maybe you are actually on the best path already? An e-foil have a very little wet surface contact and little drag from water when the thing is flying above the water and should need less power, and possible even be able to use less cells while still getting decent ride time. Maybe I keep a close eye on your project and just see what works and how you manage the foil aspect.
Have you seen any math for e-water crafts like e-surf/wake/long/e-foil what kind of power is needed to have fun? And I don't mean fun in the EU 250w category - that ain't even close to any fun, I am thinking more like upgraded sur-ron fun.