I've had the Nicro vents for the past 15 years and haven't had a problem with water entering. The old ones had a plug, which I used during an offshore passage or two. During coastal sailing, however, I have encountered some higher than expected winds on a number of occasions, with a heavy spray coming over the bow while the vents were in place. We have never had any significant water ingress and no longer worry about closing off the vents.
We've replaced 2 of the solar vents that are installed in lazarette hatches. The newer models have a built-in closure that is supposed to handle any green water issues. We haven't felt the need to close them, but haven't checked carefully to see if any water has gotten through when green water swishes past the coaming.