1) someone has removed the start in gear prevention device. This is sensible as a short term fix to the problem your having (to get you home) but it is a safety item and you should look to reinstall once the engine is repaired. The alternative of revving the engine and engaging gear is horrendous as it will destroy your gearbox.
2) Your engine is off a cylinder. To eliminate a problem with the boat side of things use a different fuel tank and line if you have one, this will eliminate fuel contamination. I also assume the fuel tank vent was open if you have a manual one.
3) Start the engine and warm up then stop it.
4) Remove the bottom plug cap and start on top cylinder only. Then repeat by removing the top cap and start on the bottom cylinder only. Whichever of the pots that doesn't run, you have identified where to begin looking.
5) Do the easy things first. Check for spark on both cylinders. If you are missing a spark, check again using new plugs. If you are missing a spark it is likely to be an ignition coil or points and condenser on an older engine. You might need help if it is ignition. You might want to disconnect the kill switch to eliminate that too. Make sure plug caps are in good condition, if they are the screw in type, these can be swapped over when testing to eliminate that as a problem.
6) Next, rotate the flywheel until you can see the bottom cylinder is at the firing position (bottom dead centre) and squeeze the fuel bulb, if it keeps squeezing and you smell lots of petrol, look at the carb, if fuel is coming out of the throat or the drain the float chamber is flooding and the needle valve is stuck open or shot. If not, back off the fuel pump bolts and see if fuel pours out of the middle hole, if it does, it is the pump diaphragm. You may need to have a spare pump/block gasket as these can be worn out or tear taking the pump off.
7) if it is none of the above it is time to clean the carb out. Not too difficult on one of these, but if the main jet won't come out, get it cleaned in a sonic bath, the main jet is easy to break on these.
8) if you still have not identified a problem it is either the reed valves or on this engine the bottom crank seal, the top seal is less likely to go.
If you get stuck PM me.