Some hybrid cars run series and series/parallel motors. It has to due with hybrids using gasoline engines like regular cars and electric motors. The two motors have to be able to work together for the car to run.
Just one word: Depends. If you need more power and you are not worried about wind against, you can take a parallel ... But everthing you depends for the purpose of the motors.
motors should be parallel to be able to spread the power equally and horizontillay creating a better force of presence behind your boat to propel the boat in a forward motion.
