Hi my name isXXXXX will assist you.
The starter has 1 wire yellow/red 1 orange and 1 large red from the battery. Some have a purple. The wires that you are interested in are the large red wire the orange alternator wire and the yellow/red start wire from the key switch. The starter needs good voltage to the starter at least 9.5+ volts when cranking. Test the voltage on the large terminal for 9.5+ volts while cranking, if you do not have this amount of voltage the starter will not turn the engine and you are loosing voltage due to a bad connection, bad battery, bad batt cable or poor ground. Do these tests and post back with results. If you have 9.5+ volts and the starter does not start the starter is bad.
Have you checked the voltage at the starter on the main terminal while cranking the engine? What was the reading?
Is this your engine serial number and if so are there any letters in the first digits?
The engine falls between 1987-92 OB527955 thru OD714106 and your number OC603377 would put you around the middle 1989-90.
now got spark to plugs but not starting there is no kill switch on boat .anything i can do to make sure fuel is getting through or pump working