I don't know the Yanmar engines, but this is pretty common to many engines. First, make sure your oil and coolant levels are normal. Now you need to replace the cooling sea water with water from a hose. What happens is that the seawater comes in (possibly through a strainer), passes through a heat exchanger, the raw water pump and sent out into the exhaust system. The heat exchanger is what cools the "fresh water" that is used to actually cool the engine. The raw water pump needs to have water flowing through it to lubricate/cool it (assuming it is a flexible impeller pump). It will self destruct pretty rapidly if it is not pumping water.
So find where the sea water comes in, disconnect that hose, make up an adapter hose that will fit what that hose feeds and has a garden hose connector on the other end. Hook up a garden hose, turn on the water, start the engine. Make sure you have water coming out the exhaust, sometimes it takes a bit for the water to make its way through the system, expecially if you have a water lift muffler. If you don't have water flow, shut down and figure out why.