Hybrid drive, JetProp

Here is a concept for a hybrid jet/propeller drive. This concept avoids mechanical gear reduction while allowing a larger propeller for better efficiency.

The motor spins a pump.
The water then flows through a turbine rotating the opposite direction.
The turbine and propeller are a single integrated part
The turbine/propeller will be designed to decrease rotation speed of the turbine and increasing torque; hydraulic gear reduction.
The turbine/propeller will spin freely during unpowered foiling

My bet is that the overall compressor+turbine efficency is way lower than 80% ( 90% in both compressor and turbine!)
A folding prop + gearbox is simpler and more efficient both powered and unpowered

You are probably correct. Part of the idea was to create something that could easily be 3D printed for the home builder.

Some of the (hoped) efficiency gains are:

The flow through the pump/turbine contributes towards thrust
The counter-rotating pump and turbine helps minimize wasted energy by straightening the jet portion of the flow.

Some other small advantages:

Corrosion resistant construction
No need to seal the drive train other than the motor
Fairly simple construction for a DIY builder

The big hurdle would be designing and refining the combination of pump/turbine/propeller to make it work acceptably well.

A folding prop could be integrated into this concept.