I had a reputable dealer recommend I have the engine treated for carbon build up. I keep my fuel treated and use 100% synthetic oil. What is this and how is it done? The dealer said this is part of an upper tune up. Also is it recommended that the O rings in the injectors be changed?

'Carbon build up'?? Well, I'm still in the 'early years' of bass fishing and my first rig BUT, speaking regularly and often with my marine mechanic (in my signature), he has never mentioned anything like that.
But, on the trailer tires AND wheel bearings - BIG TIME YES!!
Think my tires were the originals, lots of tread left but, my mechanic pointed out, I had dry rot going on and needed to think about new ones. Did that. Had the wheel bearings checked and re-packed with grease, not just to check for grease but to ensure the bearing buddies had not or were not leaking. I was terribly afraid, since I/my marine mechanic had never checked them, it would look like 'play dough' in there.
Thankfully, that was not the case but, now with new tires and fresh grease on the axles....have peace of mind.
The one thing I don't want to ruin a fishing trip is a flat or worse, axle burned up.
Just routine things....but sure do make a difference.
Have to agree with previous poster as well. TOW and semi-regular use is VERY VALUABLE.
If nothing else, use the mouse ears and water hose and just start and run for a short period.
Boats it seems are like RV's. If not used, they deteriorate and fast.
On the 'carbon build up' thing, think I would get another opinion and ask the dealer 'WHY'?? Why do he/they suspect that?
God Bless

2 strokes don't burn 100% of the oil in the cylinders which over time cab leave a bunch of carbon. Get enough and it can seize the rings or even worse start to burn like a glow plug and detonate a piston. Older carb motors need it more than something precise like dfi but even with dfi it's not a bad idea to do now and again especially if you idle around alot.

Some guys run heavy on their fuel additives to keep it in check Would like to see the head come off one running cocktails vs one that's not of similar hours to see how well they work. Maybe a pro wrench can post their experience on that

Anything required to give me a problem free day on the lake. From keeping stuff clean to checking the tires.

I'm always checking things. Check you motor mounts, Nuts and bolts. if you have trailer breaks, fluid levels. battery fluids levels. nuts on battery connections are tight. Put soap in the bottom of the bilge area to help keep gump from building up in pumps, i.e. live well and bilage pumps. Make sure you have spare fuses.

Mercury I run 1 oz of Quickcare and 1 oz of Quickleen to every 5 gallons of gas.

I have a hard time believing replacing an impeller 1/year in these clear water/rocky lakes of Texas is necessary.

I'm from Louisiana and have driven boats through mud and sand my entire life. Have worn down props before, and the skags on our motors were almost always shiny, but only remember replacing impellers after prolonged storage.

Used the boats for duck hunting and fishing and in both cases most of the time it was necessary to run in extremely skinny water with muddy or sandy bottoms.

Used a 1979 Mercury 50 probably for 15 years and only replaced impeller one time. The stator went out and the boat stayed parked for a year. Just figured we would change it while we were doing the rest of the repairs. Brother-in-law had a Yamaha 115 that we also used and abused for 15 years in the sand. Only thing he ever did was changed spark plugs and lower unit oil.

Maybe we were just lucky???

I guess you forgot about the other half of the state?

Everything mentioned above plus add a good coat of marine wax and regular cleaning.