Do not sail th C14.2 mod 1 with an open hatch. If you capsize the forepeek might fill with water , the boat will float as it has positive floatation under the seats. It would be much harder to rescue the boat with all that water under the decks. The drain plug at the back of the boat is for drainig the boat incase you would get water under the decks in the bilge. When the boat is not in use leave the stern plug out so the hull can "breath" so to speak.
I would just use common sense. If the wind is light, I don't see a problem with leaving the hatch open, but if things are dicey then lock that sucker down.