"Without proper ventilation many boats can be damaged from humidity and the resulting mildew."
Here in FL the dead boats all summer frequently have big mildew problems.
If power is provided by solar a DC Ozone unit works.
A de humidifier will run about $3.00 per day , so is out for most.
Best so far is a 12 inch turbine vent , as a replacement for a high hatch.
If you are cutting a new hole , purchase a RV Fantastic fan vent (about $100 -$175)) a 14x14 hole and when the boat is underway slip on the RV fan.
The pricy version has a thermostat and will open at a set temp, A rain sensor will close it till its dry outside. Under 3A on high , really minor DC use on slow .
Not for the constantly awash deck hatch of a sail boat ,
but on a PH top for most cruisers , a wave should not take it away!
ON Our 90/90 we pulled the hatch boards and built a 16x16 box on a sheet of ply that simply drops in. Top of box is level , to make up for companionway angle.
Turbine on top, huge vent hole to companionway . and rain during a hurricane drops into the box ,and out, not into boat itself.