Your dad is a land lubber and probably has never worked with gavonic corrosion. Boats are an odd ball situation, AC, DC, corrosion, stray currents in the marina....... stuff you normally do not encounter on the land side.
If your metal parts are wasting away quickly there is a big problem. Since you have a wasted piece you can trace the electrical route. Ask yourself how would electricity (ac or dc) get to this metal part. It would get out by the water connection (electrolyte) to any other metal part in the water. also, closing a metal thru-hull does not stop the electricity. A plastic one probably does isolate the part but that depends on what else is connected to that water circuit. Example the strainer is connected by salt water to the water pump. The pump impeller is metal and so is the motor shaft which is grounded to the motor case which is grounded to the common ground point
Prepair to flame guys: It is better, on fiberglass boats, to isolate to the maximum extent possible, all your metal thru-hull items from each other. The rational is a single metal part that is not electrically connected to anything else will not corrode. By bonding all the metal parts together you are creating batteries and ground paths between the various parts. The bronze thru hull bonded to the stainless steel prop shaft will act as a battery. Two thru-hulls on opposite sides of the boat will provide a better path for marina stray currents than the salt water and the side that the electrons leave on will corrode. There are lots of other reasons and situations as well.
