The steering element is, I find, initially counter-intuitive. But my first time driving one of these boats I looked at the engine, saw how it moved where the thrust "pointed", and then it became entirely intuitive - I just needed to build the right model for how the high-level system worked, in order to get an intuitive feeling for the lower-level details.
As for the throttle, you're quite right - the twist direction to go forwards is arbitrary, but the rotating all the way in the opposite direction to reverse (which is what I wanted to highlight) makes much more intuitive sense to me than throttling down to zero, moving a lever, and then throttling back up again.
I have driven a piece of farm equipment (bin carrier) which has a pedal that works this way. The pedal is on a pivot and instead of depressing the whole pedal you press on the top to go forward and the bottom to go back. Braking is a simple matter of pressing the pedal in the opposite direction you are going (though normal mechanical brakes are also present as backup).
I found the system to be rather touchy. When coming to a stop it was quite easy to overcompensate and start oscillating back and forth. It was quite fun to drive though, and for the kind of work a bin carrier does the ability to reverse without changing gears is very useful.
Incidentally, in order to achieve this continuously adjustable drive a diesel engine runs at a constant RPM turning a hydraulic pump which in turn drives hub motors in each wheel. The whole thing makes a very distinctive sound as it drives around.
As for the throttle, you're quite right - the twist direction to go forwards is arbitrary, but the rotating all the way in the opposite direction to reverse (which is what I wanted to highlight) makes much more intuitive sense to me than throttling down to zero, moving a lever, and then throttling back up again.