In theory, none of the original drains on the 20 would seem to have needed plugs. Obviously, the cockpit drain would not need/have a plug as it should be above the waterline and needs to be open to allow free flow of water out to self-bail the cockpit. Sink drain is same, but may (I doubt it though!) be equipped with a close-able valve/seacock on the inside of the through hull to prevent flooding if hte hose were to break/pop off the thru-hull. I guess technically, you should have a shut-off on hte thru-hull for the cockpit drain as well (same reason) but I am pretty sure O'DAY didn't install one. Cooler drain (if it goes thru-hull) may have a plug in the inside of the cooler to prevent cold air/water from escaping as well as outside water from back-flowing in. The 20 did not have a hull drain from the factory, but if a previous owner added one, that one DOES need a plug (type will depend on what kind of drain was installed).
If stil lin doubt, post a picture of each drain that you found and we can comment.