No one has proven that a jet drive is the most efficient solution. In fact its the complete opposite. It requires the highest amount of power for the slowest acceleration and slowest top speed! It has been proven time and time again in the marine environment that it is in fact the most inefficient (want proof, take any prop driven boat engine, change it to a jet and see what happens). However it is simply the safest option.
An outrunner is obviously going to have more torque than an inrunner and in some ways be a better solution, but in a marine environment it will have a much shorter lifespan as the bearings and other parts of the motor are exposed constantly. Therefore the cost will add up in the long run.
Alexandre has hit the nail on the head. What the efoil community needs is an inrunner (Is is easy to cool and keep out of the harsh marine environment) size that is somewhere between where the existing ones are. Something in the 70-80mm range would be ideal.
Prop wise, why don’t you show us some of your props you have designed and built and show us how they are working in their respective environments. I’m sure we’d all love to see it.