Cooling systems on all engines are prone to blockages from coral growth to sea weed and sediment to picking up a plastic bag across the raw water intake.
First make a visual check of the water strainer to see if the engine is drawing in water to the raw water pump, the next step is to check if the raw water pump is pumping; to do this we need to remove the hose on the outlet side of the pump and start the engine momentarily to check the volume of water being pumped and that the water is not aerated and give the pump a visual inspection for leaks.
Note: Aerated water generally indicates a problem on the suction side of the pump system.

If the water flow is poor remove the cover plate from the pump and check the condition of the impeller the end plate and the cam ring which provides the tension on the impeller vanes; if there are vanes missing from the impeller they need to be found! A common misconception is that they will just pass through the system, this however is not true they generally get caught up in the intake side of one of the coolers thus restricting the flow of water; people often change the impeller but neglect the cam ring and end plate thus reducing the pumps capacity.

Then we need to follow the water system right through, different engines have different systems but basically the theory is mostly all the same. A digital hand held thermometer is the perfect tool to help diagnose problems with the cooling system by tracking the path of the raw water flow from the raw water pump follow the hoses or pipes through the oil coolers and heat exchangers out to the manifolds and through the risers.
What we are looking for is a significant change in the temperature of the water as it passes through one end and out the other end of each component of the system. It’s recommended that these components be serviced about every 2 years or 200hrs, have them acid dipped and pressure tested, whilst inspecting them for signs of erosion and electrolysis.

Take careful note of the amount of metal around all sealing faces especially between exhaust ports and water jackets on manifolds and risers then carefully replacing all gaskets and seals; don't forget to pressure test again before installation.

Check the battery voltage at the battery with a multi meter or load tester, it should be around 12- 12.8V without any load on the battery, then get someone to turn the key or start button and see what the voltage drops to; most inboard engines need at least 9.5 volts at cranking, later model MPI engines will require a minimum of 10.5 volts.
If there is not sufficient voltage, remove battery and charge or replace with new battery.

Is the size of the battery adequate for the engine?
Most engine manufactures will give a CCA (Cold Cranking Amps) rating for each engine in their range. A small solar panel is a good idea for boats on a swing mooring to keep the battery topped up.Next check the both sides of the battery switch for voltage drop then check the voltage at the starter motor solenoid, you should have the same voltage all the way through the system from battery to starter motor. You also need to make sure you have a good earth return from the engine back to the battery, check for a build up of corrosion at either end of both leads.

Can you hear the solenoid click?
Generally you will have 2 main wires down to the starter motor;
The first wire carries the battery power; it is typically the large cable. The second is the trigger wire that comes through the wiring harness from the ignition switch, it is usually a smaller wire around 2-3mm in diameter.
Check the voltage from the trigger wire with the key in the start position, this should be around 11-12 volts, however you will need to refer to manufactures specifications in the workshop manual.
Some engines are fitted with a remote solenoid, so remember to check both sides of the solenoid for power both in and out. Next try a socket and bar or tension wrench on the front of the motor to turn the motor by hand; most engine manufactures will give a torque reading for which the engine should turn by hand. Sometimes salt air or salt water corrosion on the starter pinion can stop the starter from engaging or may cause the pinion to stick half way in, which will stop the engine from turning over, If this is the case disconnect the battery & remove the starter motor for service, or replacement.

Does the engine crank over fast enough?
Check battery / starter motor and connections for correct voltage and cranking speed, refer to workshop manual for specifications.

Does the engine have sufficient compression for combustion to take place?
Carry out compression test and confer with workshop manual, remember most petrol engines need at least 90PSI of compression for combustion to take place.

Is there adequate spark at the spark plugs?
If the engine is an early model with a points type distributor; first check if the spark is present at the points by cranking the engine over with the distributor cap removed.
Note: Whilst carrying out this process check that the shaft is rotating, thus eliminating possible internal engine issues such as cam gear or drive pinion.
Eg; Coil
      Condenser
      Pig Tail Wire

If there is no spark or the spark is weak, test and replace ignition components as required.
Check that the problem is not remote from the engine by disconnecting the tachometer wire from the coil and bypassing the kill switch.
For later type electronic distributors check you will need to consult the workshop manual for specific instructions for type of system.

Is there fuel getting up to the carburetor?
Check that the fuel tap is on; check the fuel tank vent is clear, check condition of fuel lines and the fuel filter.
Note: I strongly recommend that flexible fuel lines be replaced every two years for safety reasons.
Check the fuel pump for both pressure and vacuum; the pumps ability to draw fuel from the tank is just as important as is its job to lift fuel to the carburetor.

Is the engine getting too much fuel?
Check that the fuel tap is on; check the fuel tank vent is clear, check condition of fuel lines and the fuel filter.
Note: I strongly recommend that flexible fuel lines be replaced every two years for safety reasons.
Check the fuel pump for both pressure and vacuum; the pumps ability to draw fuel from the tank is just as important as is its job to lift fuel to the carburetor.

Is the engine getting too much fuel?
Sometimes when an engine sits around for prolonged periods fuel can evaporate from the carburetor and or drain back to the tank via the fuel pump; this may cause the float to drop and the needle and seat to stick open. This will typically cause the fuel to flood into the engine. A light tap on the float bowl with the handle of a screwdriver may free up the needle and seat, otherwise the fuel system may need overhauling.

Does the engine start on ether; E.g. Aero start?
Try spraying a shot of ether down the engine air intake, if the engine wants to start on ether, this generally means that you have spark at the spark plugs, but the fuel is both old and stale or is not getting through to the cylinders.

How old is the fuel in the tank?
If the fuel in the tank is over six months old, bypass the fuel tank using a remote fuel tank with fresh fuel. Drain old fuel from the carburetor, clean up any spillage before trying to start the engine.