The starter motor tries to turn the engine without success
1. The most likely cause is reduced battery voltage, so check voltage as described above.
Engines which depend on an electric starter motor should have their own, dedicated battery.
2.Water in the engine will also stop the engine turning. Water entering via the exhaust pipe
– large following seas can force water back up the exhaust – or via the head gasket can stop the pistons moving. Trying to start the engine with water in the cylinder(s) can break the crankshaft, pistons or connecting rods.
Exhaust water traps, goose necks, check valves and anti-syphon devices can all reduce the chances of this occurring. If water is suspected, remove the injectors and try turning the engine over.