#3: The Engine Is Overheating
The needle on the temperature gauge is rising. This almost always means you have a lack of water flow in the cooling loop.
Outboards, most small inboards and I/Os don't have radiators like your car, and instead use the water they are floating on to cool the engine. If that water stops flowing, the engine heats up and can ultimately fail.
Trace the source. In a vast majority of cases, the problem is an obstruction in the raw water intake - like weeds, mud or a plastic bag. Locate the intake and clean it out.
A loose hose clamp or a split or burst hose can also slow water flow, and it can spray damaging moisture around the engine.
Regularly service and replace the impeller. Also look at the condition of its housing. Scarring or pitting of the metal housing can cause even a good impeller to lose pumping power.
Make sure you or your mechanic checks for corrosion or blockage in the exhaust system. Every so often, have the exhaust risers and associated components opened up for inspection. Engines with closed-loop cooling systems (essentially a radiator cooled by raw water) have additional issues such as internal clogging of the heat exchanger. Beyond ensuring that the coolant reservoir is full, periodic maintenance is the key.
Soft wire or rod to snake intake clogs.
We hope this List assisted you in your ability to remain calm in the face of possible "on the water" problems, helping to insure many a successful boating adventure.