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Outside pressure and assistance would certainly help, but change can come from within too. Ocean pollution is diffuse and hard to see the effects of, especially when you're participating in a developing economy and you've got a lot more pressing needs. River pollution is easy to see, and results in clear health hazards.

One of the sparks of the US environmental movement was when a river caught fire because of all the crap that was in it. [1]

Agitate towards cleaning up rivers and other water bodies; sewage, solid waste (trash), and industrial waste are pretty common, and they're all manageable; it just takes capital, sustained effort, and some amount of enforcement. Dumping is visible enough that enforcement in the court of popular opinion can be effective enough, when the courts of law are not enough.

Having clean(er) water will help with all sorts of health issues, the economy, and international relations (competition over water sources is lessened if downstream water is cleaner and more usable)

Keeping plastic out of rivers should also keep it out of the ocean; unless you just collect it all, and then ship it out on a barge and dump it; but even then, having built the collection apparatus, you'll be in a better place to deal with the waste responsibly later.

[1] https://en.wikipedia.org/wiki/Cuyahoga_River