Prop damage, something fouling the prop, dirty air filter, lazy turbo.
-Since you are out the prop fouling may not be the case or would have been see.
- Prop damage not always visible. Damage can cause a sharp increase in engine loading.
-Dirty air filter can reduce the air available to the engine so the fuel is not burned fully producing smokier operation.
-lazy or seizing turbo. turbo can suffer damage from a poorly designed and set up exhaust system which allows sea water to back up into the turbo. It will rust which can seize, or partly so, the turbine but also the rust will increase the clearance reducing air delivery so producing more smoke. That might account for the change in sound.
The exhaust and turbo are going to take some work to examine and determine if that is the problem. The others can be checked while the boat is out although the prop will have to be checked by a prop shop as I've seen tweaks that are difficult to spot visually yet can cause trouble.
There were a few years, Mainship early production, that had real problems with exhaust designs and flooded turbos. Get yours checked.
Go to Seaboard Marine and do some exhaust system reading and then look at your system. All the info is applicable regardless of engine mfgr.
Tony's Tips - Seaboard Marine
Understanding Low Power Troubleshooting - Seaboard Marine
You might also look at Boatdiesel.com This subject has come up many times. Although different vessels the theme is the same/similar, overloading of the engine and or exhaust system problems.
Just a few thoughts to get you started.