First thing is to read through this information very carefully several times to make sure you understand everything.
Check the values for CRUISE_THROTTLE and CRUISE_SPEED. Make sure they aren't too high or fast.
Then put your boat into STEERING mode and see if you can drive it around like you do in manual mode. STEERING mode uses the same code as AUTO and this makes sure you boat steers in the correct direction whilst in STEERING/AUTO modes.
If you still have problems please attach a log file.