The engine may be rusted inside. I would hand crank for a few minutes with plugs out and with a tablespoon of Marvel Mystery oil in each cylinder. Also I should imagine it needs new electrics - plugs, rotor, points, condenser, HT wires - and all electrical terminals tightened. Check that the shaft is not stuck by turning the propellor by hand (with the plugs out). If the boat is in the water, this will entail a little swimming. If the engine fails to fire after about 20 seconds, stop cranking because the exhaust lift box will fill with water which could back up into the engine - as you say, it is a good idea to start with the cooling water turned off. Make sure the fuel reaches the carburetor and that it contains no water. In Spring, I check that the choke closes properly, then give the air intake a shot of ether "engine starter" before pressing the starter.