What is the specific model of the engines? When looking up the specs I see the ones I can find are 3,000 rpm rated but with turbos. Is yours rated at 2800 or 3000 rpm. The N/a units I see are lower HP and 2800 but need it confirmed. 4 cylinder or 6 cylinder?
Does it make rated rpm by a digital tachometer, not the dash tachs? E bay for ~ $20 for a laser pointer digital phototachometer. Get one, use it, as you may not be running at the revs you think you are and that might be part of the problem.
One reason you may be seeing more black soot now vs several years ago is the weight of the boat has increased whether you realize it or not. That may mean the props are no longer quite right for present operating conditions.
The other I can note is your choice of boat speed. Not being run at either displacement speed or at planing speed. You may be putting the hull at a trying to climb over the hump speed which will put increased load on the engine without it getting the air it needs to support that level of fueling.
Back off the throttle a bit and see what happens. You may find that backing off a bit produces a lot less smoke.
THe next time the boat is out have the props checked at a shop for any tweaks or damage and consider having some pitch removed. Even if you run at the same boat speed the engine will rev more taking in more air for a cleaner burn and less per cylinder firing load.
Tony has far more experience than I but I have been through something like this. I agree about the engine age being a factor as mine is even older but a lot has to do with how you run and set it up.
The one big difference between our boats is I have a dry stack and I could SEE the difference immediately after my prop. downpitching. Same boat speed but less smoke because at that same boat speed my engine ran 150 -200 rpm faster yet still well withing its capabilites at less loading per firing. Just more firings. The engine burned cleaner. Now I just run slower.
At your indicated cruise rpm you could easily run up another 100 or 200 revs to keep the boat speed although I think that is part of the problem. You can however make life easier for the engines.
Hope this makes sense.