Safety note for gas engines: before you crank a gas inboard or sterndrive, run the blower for several minutes to clear any fuel vapor from the bilge, and check the bilge for a fuel smell first. Cranking a gas engine in an unventilated space with vapor present is a real explosion risk. This applies every time, not just after a fuel spill.
Nothing happens at all: no click, no lights, nothing
This almost always points to the battery or a connection ahead of the starter, not the engine itself. Check that the battery switch is actually on. Look at the battery terminals for corrosion or a loose connection, a surprisingly common cause. Check the battery's resting voltage if you have a meter; a battery that reads very low may just need a charge, or may be at the end of its life. Also check any main fuse or circuit breaker between the battery and the ignition circuit.
You hear a click, but the engine doesn't crank
A single click usually means the starter solenoid is getting power but the starter itself isn't turning the engine over, often from a weak battery that has enough charge to click the solenoid but not enough to spin the starter under load, a bad ground, or a failing starter. Repeated rapid clicking is more specifically a sign of a battery too weak to crank, or a bad connection somewhere in the starting circuit.
The engine cranks, but won't fire
This is usually fuel or ignition, and a few things are worth ruling out first. Sterndrives and many inboards have a neutral safety switch that stops the starter or the ignition from firing unless the shift lever is confirmed in neutral, so confirm the shifter is fully in neutral, not just close. Check that the kill switch lanyard is actually connected; it's designed to stop the engine, and a disconnected or damaged lanyard will prevent it from starting at all. From there, think about fuel: old fuel, especially ethanol gas that's sat over a winter, is one of the most common causes of a no-start on an engine that cranks fine. If the boat has sat a long time, or if you smell strong fuel, it's possible the engine is flooded from repeated crank attempts; give it a few minutes and try again without pumping the throttle, per your engine's flooded-start procedure if it has one.
It starts, then dies right away
An engine that fires and then immediately stalls is often not getting continuous fuel delivery, or is losing cooling water flow and shutting down on a safety cutoff if your engine has one. It can also be an idle or choke issue on carbureted engines. If it starts and dies specifically when you put it in gear, that points back toward the fuel supply struggling under a slightly higher load, or a shift-related safety interlock.
Battery and charging in general
A lot of "won't start" calls trace back to a battery that was never really at full charge, or a charging system that wasn't keeping it there across a season of light use. If the boat starts fine after time on a shore charger but struggles again after a few days at the dock, that points to a charging problem or something drawing power while it sits, not the starting circuit itself.
When it's time to call someone
If you've worked through the checks above and it still won't start, or if you smell fuel and aren't comfortable troubleshooting a gas system yourself, that's a reasonable place to stop. See boat diagnostics for a systematic, on-site read, or marine electrical if it's clearly battery or wiring related.
Getting ready to launch for the season and want to catch this before it happens? See spring commissioning and the haul-out inspection checklist.
Still won't start
Text a photo of the panel or engine and describe exactly what happens when you turn the key.