Engine Cooling Issues
Jackie, I think Bob's idea of a diesel is a good one. It's more complicated than a 2-hour panel discussion at the OYC. In addition to the cooling system, there's the fuel system with its filters and injectors (and how to bleed them). There's also the electrical system, which for a diesel might be simpler than for an gas engine - if you can hand crack the thing to get it started. It doesn't need electricity to run; that's why the "kill switch" closes off the fuel supply instead of disconnecting the battery.
A simplistic look at the raw water cooling system: It's an open loop, with one end being the intake sea cock somewhere in the bilge under the engine. From there a hose to the raw water pump, which forces the water through the engine block and head to cool it - there's usually a thermostat in there to control the flow and keep the temperature regulated.
After the water passes through the engine, it's introduced into the exhaust gases in the water lift (the plastic flower pot you mention). The combined exhaust gases and cooling water are expelled through the exhaust fitting on the transom.
Each one of these elements can fail. There are several severe results of that failure. 1) The engine will overheat, seize up, and be destroyed. 2) Without the cooling water being introduced, the 1000 degree exhaust gases will melt the exhaust hose, filling the boat with exhaust gases and water entering from the transom fitting. 3) Since it's an open look: ocean-to-engine-to-ocean. If some fitting/hose/water lift fails, you'll sink. The sea cock on the intake will close off that end. The exhaust end doesn't have a sea cock, so if something fails after the cooling water exits the engine, things can get serious very quickly. If a hose fails, the water pump will continue to do its job, filling the bilge. If the water lift, exhaust hose, or the clamps holding the exhaust hose to the transom fitting fail, there's the possibility of water entering the hull that way, especially with a following sea.
When I sailed a boat with an inboard, I kept a wooden plug sized to jam into the exhaust fitting on the transom. In a Coastal Cup race when my water lift failed (and it was stainless steel) I used that plug to keep water out from off the Sur Coast to Catalina Island. Needless to say we didn't motor any of that distance! Sometime I'll regale you with all the ins and outs of that story.
So, while you're getting the water lift replaced, be sure to check out the entire exhaust system, including all the clamps and the hose. In an old boat, all of those things probably need to be replaced. And, don't forget to service the intake seacock and check that hose and clamps.
Pat