You probably saw my post re the FRAME_CLASS param in the other thread but in any case, that will arrive with -rc4 in a few days.
Here’s the wiki section for the various flight modes: http://ardupilot.org/rover/docs/rover-control-modes.html
There’s no OFF-BOARD mode… perhaps you mean Guided?
So at the moment at least, HOLD mode just turns off the motors and moves the steering to the middle. We don’t have a manual-ish flight mode that will hold position. You’ll find with a boat that if you use Guided mode to make the boat drive somewhere, once it gets there it will hold position, and in Auto mode it will do something similar after it reaches a waypoint (if you add a delay to make it wait there a while)… but we don’t have a manual mode in which you drive it around, and when you release the sticks, it fights against the water flow. Someone else has asked for that so I think we will add it in a future release. In fact, I have created an issue here to capture this request: https://github.com/ArduPilot/ardupilot/issues/7312