Channel 1 for the speed controller
Channel 2 for the rudder.
You won't do any harm trying it out for yourself - plug in and see which stick movement activates rudder and motor. Keep switching the servo plugs around until you find the one you want.
Actually I use Channel 4 on mine for the speed controller. This is the left-right movement on the left stick, and is sprung to the centre, so you can always find the stop position. It also means if you drop the transmitter the boat stops rather than carrying on at full speed
Greg