Imagine resorting to the netflix adaptation to prove your point
I'm just resorting to logic. If Zoro wasn't the first mate, this scene wouldn't have been approved.
The live action also says the schibukai were the rulers of the sea instead of the yonko
Zoro is the combatant, like it or not. And Robin is the archeologist which also has nothing to do with the sea
Even if it was true that all the strawhat jobs relate to the sea somehow, that doesn't mean all of them have to