A25 aft-cabin drain?

[Sunset]

Unable to remove the stripped drain plug from the underside of the hull, I have used my vacuum hand pump to remove the accumulated water from the deep bilge in prep for launch. Yesterday I discovered bilgie water under the floor board of the centre panel in the aft cabin but after sucking it dry it continued to receive a limited amount of water from a 1-cm drain hole located in the fwd part of this area. The floor of this area is substantially higher than the lower area of the bilge so I want to know the purpose of this hole and the source of the (minor) inflow. It should be dry under there for some storage!

Anyone?