Well, I guess... "ALL" should read, "all air vents and small openings" leave one or two major opening available for water to get out. Think companionway only, but leave forward hatches and roof vents plugged..... On a sailboat, I would close the companionway and put the top batterboard in.... things like that. It would amaze you to see the quantity of water that can get into a small louver type air vent typical on many boats. A bunch of small leaks add up very quickly.

Something else you have to watch for is making sure you get air distributed for and aft. You don't want the bow to come flying up and leave the stern down.