Outboard question

Don't count me as an expert on gearcases, some random thoughts

On the older ones with the horizontal split on the very lower half, as I said, there is at least 8 ways to get water in, and a machining misfit between the two could be an issue I guess

1,2--the filler and drain plugs
3...the little screw in the very bottom that holds the shifter fork in place
4...The rubbe snake seal around the split
5....The O ring around the boss holding the prop shaft bushing and seal
6--the prop shaft seal and if you want to count the lip and outer seal ring separate, that's two instead of one LOL
7---the shift shaft seal
8---the prop shaft seal
Examine both prop shaft and shift shaft for roughness and corrosion.

There's a fitting you can buy or it comes with the pump style gearcase oilers that can be used to pressure and or vacuum test the gearcase, so you can put a few pounds of air in there---I use 5--8, and dump the thing in a tub and look for bubbles
With vacuum, pump up a vacuum and look for gauge movement.

Someone online suggested "the right stuff" gasket seal for the split case seals. Part of the problem is china parts. I've ordered china carb float bowl gaskets that were about 20% !!!!! of the thickness of the originals. If you are VERY careful, you can often re-use those.

I don't know if you are aware, some models of the smaller, older engines do not have the oval/ rectangular access plate to disconnect the shift shaft, some of those you can disconnect by partially dropping the gearbox, and with the shifter in ?? R I think, you can get to it, but other models, you MUST pull the power-head out of the upper leg. THAT is PITA, because you must burn that base gasket. It is complicated, I cannot imagine trying it with RTV or other filler

You may know this, the gearboxes can freeze and break. Be darn sure that in the fall, before cold weather, you either check them or drain them and refill. As I said earlier, if you can store it vertically, un rotated for a time, the water will separate and be easy to see