Looking at an 18' Sea Craft, ... need some help...

[Capt Chuck]

Welcome to CSC Virgin Islander

There's a pvc pipe tunnel (actually, a half moon shaped piece of 3" pvc)that runs along the keel and ends about 3" short of the transom. The pump just fits into that little space. ... I'm guessing this is a normal part of the contruction?

The half moon glassed PVC running fore and aft was normal construction in '78. It really serves NO structual purpose. Channeling the water aft and preventing water ponding in the center under the fuel tank seems the most logical. The old wood glassed center stringer (still apparent in most earlier models) was meant to "Beef Up" the keel as most boats had roller type trailers before the float-on bunk types hit the circut

BTW: I cut mine away to install the thru-hull transducer @ centerline and complete the transom inclose.
Good Luck with your refurbishing !