If the you hear the solenoid click when you turn the key but the starter motor doesn't turn at all the videos listed on this page should help you to diagnose and fix the problem.
There is a problem with the boat's battery
The boat's electrical system starts and ends with the battery. It is important to check that the battery is fully charged and in good health before starting to diagnose any electrical problems.
The starter motor is faulty and doesn't spin
Starter motors can fail for a variety of reasons from worn out brushes to a burnt out winding.
When working on an outboard it can be handy to have a remote stater so you can watch the engine cranking rather than being up the front at the key.
Freeing up a seized engine
An engine is said to be seized when it is physically unable to turn. This can be due to corrosion, a bent conrod, bad bearings or a number of other causes.