Sheets are the lines that control the sails.
Anybody who does not button up their companionway in a blow is just foolish. We slide it closed if we are taking any water on deck as a general rule.
That said, when I was capsized in a hurricane off Fiji some years back, it tore the secured companionway hatch right off the boat. It ripped apart a 2" galvanized pipe dodger frame, broke the main boom in two places, ripped off all the cockpit locker hatches and a dozen more bits and pieces. If you are gonna go offshore then you do the best you can, but none of us can afford that 100% bulletproof boat, if such a thing exists.
A little bit of water leaking in around a secure companionway isn't really something to worry about as one should expect water in that area anyway, just from crew passing through, if nothing else.