Drain Plug
OK guys, while I am not an expert on the 37ft'ers our 45 Convertible has the same arrangement. A large thru hull plug in the forward bilge just aft of the holding tank. This plug is just as Jim inferred, for drainage during storage. If you look at your boat from the side after layup you will notice a VERY slight bow down attitude. Why this is the standard manner I have no idea but it seems to be. And as such any moisture, rain runoff, snow melt or water residue needs to drain out during layup. And I think if you all can somehow get into your bilge areas near the "bulkheads" you will see lumber holes. There is absolutely no way for the bulkheads to be solid, think of all the wiring, plumbing, and misc holes added for extras throught the life of your boat, theres all must have space to prevent chafing and breakage, therefore you all have some type of "lumber" holes in your bilges. I know my boat has them, I sent my little nephew into the bilges just last fall to make sure they were all open, and he found many of them fouled with trash, carpet fibers, dropped screws and bolts.
You can seal up those holes but think of it this way....Hatteras builds a quality boat why would they put a hole in the bottom unless they thought it was for a good reason. A better idea is to replace any broken plugs (they should be standard SAE threaded stock) and make sure you remove them every winter and them reinstall them in the spring.
I know mine do some good, there is usually a wet spot under the hole in the spring. Not a lot but it keeps mildew and moisture out of your boat while it's layed up.