I installed a Nicro vent in the 4" port atop my vberth. It allows light through the transparent center and has a sliding baffle that can be used to adjust or close it if need be. I just glued it in place with 5200 as it fit pretty snug.
Looking at the design of it I think it would be a pretty bad day if water
was working its way through the vent into your boat
as it has a trap type setup. I think near submersion would be needed if vent were left open and if that is the case you would have bigger issues to worry about.