Something Very Old Fashioned, but Important
Once upon a time, before most of today's sailors were born, boats did not have knotmeters, or at least not inexpensive reliable ones like we have today. An important part of navigating a boat was to create a table of boat speed as a function of engine RPM. If you knew haw fast you engine was turning, you had a pretty good idea of how fast your boat was moving through the water.
This is not something we need for navigation any longer, but it is still a valuable diagnostic tool for sorting out what's going wrong if your engine isn't performing as it should.
Start with your engine running well, and a clean bottom and propeller. Using either a known distance run, or an accurate knotmeter, run the engine at various RPM values, and record the resulting boat speed. Save this information for reference when you have an issue.
Let's imagine you have a performance problem with your boat. Your engine is just not running up to the same RPM values it used to. You might have an engine problem, or you might have a fouled propellor, or bottom. How do you know? If you have the table of the boat speeds and RPM values you SHOULD have, it is easy.
Compare the boat speed at the maximum RPM the engine can reach. If the boat speed at that RPM matches the one you recorded before, you know that the problem is the engine. If the boat speed at the same RPM is now lower than it used to be, you know the problem is not the engine, but something else further back in the drive train.
On the other hand, if the engine IS reaching its normal RPM, but the boat is going slower, then we know we have a prop or bottom problem.
Being able to sort out which end of the drive train is causing the issue can save a lot of trouble and fussing.