M_Boats: Through hull holes?

As I am stripping paint off this M17, I found a couple through hull holes that have been fiberglassed over and look to be done well.  They were up front, so maybe were for depth instruments in the past.  In the rear, about 6" forward of the transom, there is a through hull still there.  It is capped with pipe plug fittings inside.  I don't like the looks of some previous fiberglass work around it, so I am going to remove it and properly repair it.  But this made me curious; what would it have been used for on an M17?  It's not like one needs cooling water.  Do any of you have through hulls in the bottom of a small boat?