To troubleshoot a Yamaha outboard boat motor if a starter doesn't work, make sure that you're using a battery of suitable capacity and that the battery isn't low. Check the battery connections. If they are loose or corroded, use special tools to tighten them. Get rid of any dirt or rust in the battery terminals.
Make sure that none of the fuses are blown. If they are, the engine may have experienced an electric overload. Install fuses of the appropriate amperage. If those steps didn't help, contact a professional to examine the system.
If the engine on a Yamaha doesn't start, but the starter works correctly, the gas tank has fuel. Replace the old fuel with fresh fuel, especially if the fuel is stale or has been contaminated by other materials. Make sure nothing is stuck in the fuel filter, and clean it if needed. If you suspect the fuel pump doesn't work, hire a professional to examine it. Examine the spark plug, get rid of any dirt on it, and make sure it is the correct type for the engine. Problems with ignition wiring can also result in engine malfunction, so make sure that there are no signs of breaks or wear on the wires and that they are connected tightly.