3/8ths? Thats way too small to be a splashwell drain.
A splashwell drain should never be plugged, its intended to let water OUT of the boat that splashes into the well (hence SplashWell and Drain).
I dont know where that hole is or what its for, but it isnt a splashwell drain.
Post another pix, from further out to get perspective. If the hole is hard to see in the pix, put a circle around it or use an arrow to point it out.