Ardurover won't decrease in speed

Hi I am using and Ardurover firmware for my boat. My apm is version 2.5. However, the problem is during auto mode because the speed of the boat does not change. I tried using different value of the cruise_speed and cruise_throttle. However, the result is still the same.
I actually experienced a lot of problem regarding the auto mode:
First, when I was testing it out using the command do_set_servo, the motor spins. Even though there is no set waypoint, the motor is still spinning. It seems like it want to go somewhere.
Second, it doesn’t go to its home when return to launch is commanded.
Third, and most importantly is that I can’t seem to decrease of speed of the boat in auto mode.

My boat works perfectly fine using manual mode.

I hope you can help me regarding my problems.