I will be doing my first winterization on a freshwater outboard engine. The engine in question is a 2015 Mercury 115 Command Thrust 4 stroke. The boat is going to be stored in underground storage (old limestone mine). Temps are consistently in the low 50's and humidity controlled at 60%.

In my research it seems there are two different methods of winterization.

One group says fog the spark plugs and flush water intake with marine/rv antifreeze.

The other group says fogging and antifreeze are not needed.

Naturally I figured the only way to find out the correct answer is on Sig Forum.

My boat with twin Yamaha F250's spends its winter either on its trailer in my driveway, or on its lift at my beach house. Winterization of the engines consists of a final flush with salt-away, and leaving them tilted down over the winter. I do make sure that the last tank of fuel I run is non-ethanol to prevent any fuel-related issues from storage, but my boat typically sits only from January until march, so even that is probably overkill.

A modern 4-stroke outboard is no different than your car engine for the most part. You could park your car in that mine, and come back in a couple months and expect it to fire right back up. Your outboard is the same.

Stored vertically, all the water should drain out so no antifreeze is needed.

Run it with stabilized fuel.
Change the engine oil and gear oil in case there's any water in either.
Fog the cylinders.

Fogging is optional, but the antifreeze flush is not necessary as long as you at least flush with freshwater and drain properly.

First year for us with outboard as well after 23 years of I/O. On delivery I verified with the marina owner that just making sure it was fully trimmed down was all that was needed from a freeze standpoint. Not planning to fog since our down season is only 4-5 months, but will sue stabilizer just because.

I only run non-ethanol in my 2-stroke outboard, but winterization simply consists of parking it in my garage (unheated, and yes it gets below freezing in there for most of the winter), and conducting my annual lower unit gear oil change, which is really just preventative maintenance not winterization. Been doing it this way since 2014, never had an issue. No saltwater here, only fresh.

I do the lower unit check and make sure no water is in there. I make sure the gas tank is full to the top leaving no room for condensation in the tank. Put it away in the barn and see ya in spring. Never had a problem.

For the most part, only ethanol fuel is available for me during the season. At the end of the year, I run it down as far as possible. On the last trip to the ramp to get hauled, I run my 4 stroke off a 1 gallon can of ethanol free fuel to winterize it.
In the spring, the remaining few gallons that are left in the tank is pumped out and I start fresh.
As far as fogging the engine, I haven’t done it in years. But when I had my old 2 stroke Evinrude, I found that a few squirts of Marvel Mystery Oil in the spark plug holes worked much better for me than fogging the cylinders. Spring startup was a breeze with MMO.