I think you would need to do your own. This seems like a very limited market. Not all boats could even use it. In most cases for safety you still need two hands to hold a two stick transmitter while the vehicle is moving. And in most cases turning also involves controlling more than just rudder. Sharp turns in a high speed power boat usually require throttle control. All turns in a sailboat require sail control. A tug boat or battleship might get by with following a path but that's about it.
If you really want to do something like this you might try that other unnamed rc web site - it has a lot of information about open source programming of transmitters.