The water boxes on our boats are designed in such a way that a little residual water lying in them will not damage anything when it freezes. A couple quick revs is all you need. I do it every time I take it out of the water.
I had a waterbox out of the boat while upgrading my through-hull drain (scupper). Just for S&G I dumped the water into a small pail to see how much was in there. After a LOT of rolling it around, turning it, and shaking it, I probably had a couple pints of water in the bucket and could still hear some sloshing around in there when I gave up. You just can't get it all out.
My boat is stored in an un-heated building and has happily survived 10 Wisconsin winters (we've gone weeks at a time with highs of -10 deg F) with no issues.
Moral of the story: Don't waste your money on anti-freeze. Some folks do it to "feel good" but dumping money (literally) into a storm sewer doesn't make me feel good so I skip it.
The manual recommends fogging the engine through the throttle-bodies by spraying into each intake with the engine off, then starting it for a few seconds. I typically run the engine on the hose (Engine on, water on, water off, engine off. In that order), and spray some fog in while it's running. Then I shut down and follow the manual recommendation of spraying each throttlebody and starting.
(In all honesty, sometimes I feel like fogging is a feel-good thing too, but it IS recommended by Mother Yamaha.)
As mentioned, sta-bil in the gas and don't fill the tank all the way -especially if you store outdoors. When filled fully, fuel could be pushed out the vent line when the weather warms up. (With the floor removed and a full tank, you can see the air pocket bobbing around in the tank - it's not 100% opaque. If the boat is a bit nose-up - which you should be for storage - the air pocket is at the front of the tank. When it gets warm and expands it has nowhere to go so fuel is forced out the vent line.) I haven't personally had to deal with this, but my indoor storage unit seems to warm up slowly enough to keep this from being a problem.
I'm sure
@Gym will get you sorted out, but feel free to ask questions here and don't be afraid to do this stuff yourself. If you can pour water out of a boot with the instructions written on the heel, you can handle this.