M_Boats: Hole in the bottom of my boat

Hey guys, I need your help. I bought a 74 M-17 that has been neglected. On the bottom was a threaded hole about 2" across, about 12" forward of the battery and porti-potty bracket right on the center of the hull. I think it is a transducer or depth gauge sticking out of that hole. There is a round piece attached to a wire hanging out from the hole. Am I right? Kendall