I have spent the better amount of my lifetime flying airplanes. 99% of that time, 38 years, has been professionally. When I read the performance threads, I often cringe, as everyone isn't aware of all the factors that affect performance. On a non turbocharged engine, everything affects it. But some things more than others. As a generalization, increase in elevation, or temperature, or atmospheric pressure, will decrease performance. This decrease is not apparent with subtitle changes. But when reading on a forum such as Jetboaters.net, and our varied memberships lakes and ocean locations, it isn't subtitle. A guy boating in Utah or Tahoe, faces a much grater loss of performance than a hot day verses a cold day at sea level. A MUCH greater loss. It is a routine loss for the high altitude boater (or flyer). But the 192 is an engine capable of maintaining sea level performance MUCH higher in elevation than sea level. I have not run the charts, but there isn't much loss due to temperature swings on a turbo or super charged engine. For the most part, cold dense air will produce more HP than Hot thin air. But the turbo makes up for the less dense air due to elevation change. I have said many times, the 192 isn't for everyone, and it is often misunderstood by the masses, as to what its capability is, over its counterpart the 190.
Finding out what changed before maintenance to now is important. Finding out if something isn't normal also. Some have had an in compartment exhaust leak, others a dirty hull, others lost power due to old fuel, and still others had an air loss due to dirty air filter. Finding out if you have an air filter problem is easy, take them out and see if it runs normal again. Same with an exhaust leak, open the engine hatch and run it. If the power comes back, you are on to something. But the difference on a 192 on a 50 degree day vs a 100 degree day is not a severe change in performance. On a 190, it is much more apparent.
I learned 30 years ago, that you don't buy the big engine to go faster in s boat, you buy that so you don't lose as much when loaded heavy or on a hot day or higher elevation. The biggest restriction in speed is the hull and pump, not the engine. When you have the small engine(s), even subtitle changes in elevation and temperature, will be more noticeable than on the bigger engines. The 192 erases some of these changes due to its ability to maintain sea level intake manifold pressure to a higher elevation.