If your boat heads into the wind for the majority of the venting time, and your vent if mounted forward, you will want to set the fan direction so as to blow into the boat. If the prevailing winds are from the aft (my boat, in a slip) you will want the vent to blow outward and exhaust the air (the sliding hatch provides the makeup air). I only suggest this so that the fan works to assist the already-established natural airflow, instead of working against it. Symbiosis and harmony in the universe.
To tailor for individual conditions it is fairly easy to reverse the flow, of course, by changing the fan blade assembly from the clockwise to CCW one.