There is a fluid drain plug on the starboard side of the engine closer to the rear lower part of the block. Try opening this and seeing if water flows out properly. You can stick something into this hole to break up possible junk that is just near the outlet. If not, you may need to flush the system. Check the
exhaust elbow. They crud-up and degrade after 20 years. Check the thermostat and make sure it is opening at close to the right temp for a raw water cooled engined.
Paul L