mode 2 is the generally accepted format here in uk, that's throttle on left stick, what ever rc discipline. Mode 1 tends to be USA, throttle on right. Most transmitters will have the throttle un sprung, so low sick will be zero throttle, with rc aircraft you obviously don't have reverse, and the other stick has sprung to centre for other channels.
If your boat has reverse, you will need to have a sprung throttle. Lets assume its a simple operation you need, mode 2, throttle on left, up/down, there is now no requirement for the left/.right channel. On the other stick you will have your rudder, no need for the up/down on that stick, with most transmitters you can open the back, remove the spring assembly. and reverse it over to the throttle stick, so now that's sprung

I have done this pn planet t5, turboxrix and flysky, DX6i was more complicated but can be done. I made up a few dics that went behind the sticks with slots that only allow the direction of travel I wanted .
Hope that helps!