While having fans circulate the air is a good idea, you really need to get fresh air in and old air out of the boat... This is really key to keeping mold/mildew at bay.
The Nicro solar vents I use have never been an issue with regards to catching lines.
I will look into them again, SD. I was reluctant to pop a 3-inch hole in my boat an potentially introduce a leak, while I was getting mixed information about the vents getting caught by lines and tossed off the boat.
Do you remove the vents and replace them with caps when you sail or do you just leave them in? Also, how watertight are they? Some vents that I looked at have a means of closing them off for bad weather and waves breaking over the vents.