Two things to consider:
Size of sea zones has no effect whatsoever on anything. Since there are no resources at sea, the size of a sea zone is nowhere even calculated in the game.
What matters is the center points, because like for land regions, travel between sea zones is largely assumed to be from center to center (with exceptions for embarking and landing, so a next-region ship trip does not actually go via the center).
Shifting sea zone borders in within the same region (as in the Sallowcape/Ravielan case) does absolutely bugger all. Well, the center shifts a little, but it'll probably make no difference in travel times, and it doesn't change where you can embark or land, so it's purely cosmetic.
Finally, like region borders, most of the zoning was done with geography in mind, not political borders or historical importance, which can always change in the future. Likewise, the sea doesn't care where cities and strongholds are.
That said, these zone are probably not the last word and absolute perfection, I'll grant that. So I am willing to listen to input.
