Although Varuna's Origin Laws allowed the Boundless Sea and Sky Domain to exist eternally, Vaan did not doubt that Varuna's long absence had brought it endless issues.
The Lord of Chaos might have looked after the Boundless Sea and Sky Domain, but he still believed it wouldn't be as stable as it once was.
Perhaps the inhabitants of the Boundless Sea and Sky Domain were no longer united and had been divided into countless smaller factions that served their interests.
Sea God Proteus could be one of the Sea Gods looking for him in hopes of restoring the Boundless Sea and Sky Domain to its past glory. Or, Sea God Proteus could also be one of the beings looking to kill him.
Given Varuna's Boundless Sea and Sky Aura had spread to the point that other beings could utilize it, Vaan wouldn't be surprised if it gave birth to ambitious beings who were trying to gather the scattered aura and become the new Lord of the Boundless Seas and Skies.