Farr 11.6 (Farr 38)
There are a slew of risks to doing what you suggest on an engine that has been sitting for 15 years.Rock it to free the crank. Disconnect the feed to the water pump and stick the end of the hose in to a bucket of water. Do not use a water hose directly to the pump. Connect a battery and start the motor. Take it from there.
First of all the fuel in the tank is stale and bound to be contaminated. Without changing the fuel before trying to start the engine, there is a risk of damaging the injectors.
Since this engine has sat for 15 years, it would take a lot of cranking to get it to start. Cranking the engine with the raw water intake open or sitting in a bucket of water, risks hydrolock, and thereby damaging connecting rods, valves or worse.
Cranking the engine without replacing the raw water impeller risks the impeller shredding and clogging the heat exchanger.
Running the engine without changing the oil threatens to pump water and gunk into oil passages and damaging bearings, seals and journals.
I still suggest that it pays to take a more conservative approach to starting an engine that has not been started for this long.