> colonial powers effectively invented it

“In both Roman law and Islamic law, notions of a commonality of the seas were firmly established” (Id.). (It’s also weird to describe a custom of commons as colonial. European colonialism was about the opposite, turning historic commons into private rights.)

As a normative concept, you’re right, it’s new. But the notion that a great power would protect sea access for a variety of groups is old. More as a practical matter, granted—it’s hard to project enough power onto an ocean to control it.

What is the source?

Roman and Islamic law were also pretty much "colonial", even though the term is used of modern European empires, Rome was also an Empire, and the Arab Empires were also aggressively imperialist and maritime traders.