The drain exits on the port side, without any type of external fitting (it's just a hole, maybe 3/8" dia). As my project isn't in the water yet I can only speculate on backfilling, but with it's small diameter and it's location pretty high I wouldn't think it'd be much of a problem. I could imagine some water getting in if you were burying the bow that may wet things, but not to the point of filling the locker, as it'd be draining back out as well.
One improvement I'd make in the newer model anchor lockers is a bolt or latch to lock the lid!
Also, you may want to reinforce locations for two large cleats on either side of the locker if you're going to have a deck hatch, as a midships cleat will lead the anchor rode right over the hatch making it impossible to open when you have a strain on the rode...