Do not use just any automotive coolant. Lookup the specs for your engine and use the recommended coolant (yanmar will typically recommend 2 or 3 approved types). It will almost certainly still be a type of automotive coolant, but it will be speced for the engine. This is important as different coolants are compatible with different materials in the engine. If you get the wrong coolant it can be disasterous. Since the boat is new to you and you are not sure what coolant was used, I would drain and fill it. It won't cost you any more money as you have to buy a full jug of antifreeze anyway. If you don't mind spending a few dollars more mix the antifreeze with distilled water as this eliminates the impurities and minerals.