The basic job of the two inner seals is to keep a solid "plug" of grease between the oil and the water. I'll make that clearer.
The primary reason I like silicone greases for that is they wash out in neither water, nor oil. So they stay in place well. When mine comes apart for service, the grease between these seals is almost as clear and white as the day it goes in.
Ironically, the wear on the "wear out bearing" is always the worst on the innermost seal, lubricated primarily by the gearcase oil. It's counter intuitive to me at least, but grease is the more effective lubricant for seals.
Spanish Wells, Bahamas