[patmorgan23]

It would probably be a different larger nation that's geopolitically opposed to China.

[m0zg]

But then you don't need a treaty, because that nation can blow Chinese vessels out of their waters without it.

[amelius]

?

the big nation might not care if Chinese vessels are in the waters of your small nation.

[m0zg]

A big nation doesn't give a shit about a small nation unless there's something in it for them. I.e. if they're a part of some pre-existing union, or if they host their military bases, or something else. And there's zero chance a big nation will risk going to war to protect a small nation's fishing rights, unless a war was already in the cards (in which case it might be used as a rather desperate "casus belli").