I have something similar fitted on the cabin top. Lets air move and keeps out rain and so far breaking seas. Wouldn't trust it to do that on the deck of a 'wet' boat though.
Apart from the balls there does seem to be a disc that scews down with the internal handwheel to seal the final inlet to the cabin. Presumably you can fasten that down when things look grim.
IMO and having fitted out in the States I am impressed with West M.because their customer base ranges from the tropics to the very frozen north-inferior product soon gets dropped from the catalogue!
Quite refreshing that YBW has the good sense to offer a link from their homepage now.
I installed a similar vent at the beggining of the season, and it is the best investment I've made so far on the boat. It is a solar powered one that charges its own battery so it runs 24hours. Absolutely fantastic, the boat feels so much fresher especially when it's not been used for a few weeks. My ventilation was pretty bad beforehand though.