Hello.
Your terminology is a bit off, or you are not asking about a c-dory.
Tunnels are on the bottom of the boat, they are a scoop in the hull that pulls water up and allows your prop or jet to be raised to a higher position without cavitating.
C-Dories do not have tunnels.
If I had to guess I'd say you were asking about the splashwell in front of the motor with drain at the bottom. It is totally okay for this to fill with water. You need no one-way valve, nor do you need to plug the hole. It is just fine for the splashwell to take on water, it will drain just fine. The purpose of the splashwell is to contain the water that comes over the transom, keep your motor from getting swamped, and return that water to the water.
You can get worried if water starts coming over the splashwell and into the cockpit, but if that is happening you are likely doing something wrong and should stop doing that thing before you sink your boat.
I hope this helps. Please specify what your concerns are a little bit more clearly and you are sure to get a lot of good info on what to be worried about and what is normal.