This doesn't really answer your question, but I think it's interesting to note that Institutes of Justinian are doing two things here. First, they are announcing to the Romans that the seas and running waters are part of the commonwealth. Second they are making the seas and running waters part of the commonwealth. They are simultaneously making a declaration and changing the world to fit that declaration.

In short, seas were part of the commonwealth because Justinian said so. An <ahem> appeal to authority. But an appeal that Justinian made, not benjohnson.