You can disconnect the hose from the pressure side of the pump and drain the block into a bucket or the bilge. Then disconnect the hose from the "high side" of the manifold which should be the highest point of the engine. Once drained plug the hose disconnected from the water pump and then fill the block with vinegar (no mixing with water) at the manifold once full just let it sit for a day or two. Then hook the hoses back up and start the engine.
The vinegar does not stink like an acid wash does. So the boat will smell like a salad for a day or so not like a sewer
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After running be sure the water is flowing good. After running check the fitting on the manifold that feeds to the exhaust as it is the place most likely to gather debris that can restrict water flow and thus the cooling capacity.
Dave Neptune
