I dunno nuffin bout them but  is it possible that the  main battery is loosing capacity ? is there overheat protection on the motor. Im just trying to think of why it  wouldnt power on out to sea . Was the motor running  when it beached. Does the whole plot run on one battery or does the gps / steering servo have it own power supply . Just thinkin  maybe gps is ok but low or no power getting to steering servo or motor.......and it might be ok when you launch  it but peter out  after a bit.