I didn't bother drawing in the floats since the idea of a floating work deck isn't exactly novel and obviously, nothing is drawn to accurate scale... Just wanted to see if the concept was viable?
If you made a small, pontooned work deck, , arranged a battery bank towards the rear to power an overhead frame mounted (and decent sized) electric motor towards the front which turned a light turbine blade on an extended shaft that was immersed in the water, housed within a series of adjustable, progressive nesting cylinders that could be adjusted to optimal depth, couldn't this work?
It would be pretty stable and if smartly designed, light enough to maneuver with a push pole. If you wanted to get real clever, you could probably make the whole system light and modular enough to disassemble/reassemble into man portable components.
It could be anchored at four points. With the electric motor, it wouldn't be nearly as loud as a gas powered engine- damn near silent. One could use all standardized components to make it. If they make electric motors strong enough to move a EV converted Volkswagen @ 50 mph, they'll definitely be strong enough to generate some pretty significant force on a turbine blade. The only machining that would need to be done would probably be the shaft, some couplings and some gears, then just some basic math to figure out weight/buoyancy/displacement issues and whatever minor engineering math had to be done to account for operating forces. You'd have to waterproof some stuff, too. Intuitively, given what the components would probably cost, it probably wouldn't even be that expensive to make something like this.
Since the propulsion arrangement is directly overhead and unobstructed as opposed to deflected, you'd probably need much less aggregate energy to move the same amount of overburden that a prop wash would move- especially considering you'd be in really shallow water and imparting the force very close to the bottom anyway.
One couldn't take it out on a day with 6' groundswells and obviously, the rough sketch doesn't convey what would go into making the rig stable in a surf zone but on a relatively calm day, it would probably be more than stable enough to blow away some overburden.
