I find that sometimes not all the water comes out of outboards when in the vertical position. I.e. I have tried to drain outboards that way but have then had water spill out upon carring them. I would reccomend tilting the engine into every possible position you can, up, down, halfway (if possible), and steer straight, left, and right at each position, to maximizewater drainage. Also, lift your trailer hitch way up (but don’t smack the motor into the ground while doing that) and lower it way down. Following all that, if you have access to compressed air, I would suggest blasting out the hole where cooling water should exit, as well as all the open holes you can find in the gearcase. I think there should be a teeny tiny hole (or two?) in the gearcase (maybe front left?) for water to drain out, alongside the obvious bigger holes. It would be a good idea to make sure the little hole is not plugged with debris as it lets water drain out.
As other members have said, a gear oil change is a great idea. Basically, you put a wide pan under the motor to catch the old oil. Then undo the 2 slotted screws on the gearcase. Let it drain for awhile. Don’t let insects investigate where those screws were. Then, pump your new gear oil (go for one that specifically says it’s for outboards, unless you have an electric shift motor then you need a more specific oil that still is for outboards) from the bottom up. Go slow so you don’t make tons of air bubbles. Then once oil comes out the top, put the top screw in. Then quickly replace your gear oil pump or tube with the screw. Youtube might make this easier to understand.
Also, some people like adding a little two-stroke oil or fogging oil into the spark plug holes to prevent moisture damage to the cylinders. Then maybe spin the engine over 1 rotation so the oil covers more surface area.
