It sounds/looks like the drainage is adequate. Clapets might be effective in a following sea, if you can install them so that they will always open freely to drain.
I've gotten water in my cockpit from waves breaking over the bow, my drains don't appear to be any larger than yours and the water drained pretty quickly.
As Killarney points out, you want to keep water out of the cabin. I would want a method of keeping the hatch boards locked in place.