Japan research team develops plastic that breaks down in seawater

Excellent news.

Good.

But the first step is not to throw trash in the nature, and many countries have still such a bad habit

Well, that's in fact additionally produced plastic, on top of that which already exists and also is further produced and doesn't break down in seawater. The only and still more theoretical way would be to stop producing plastic and to recycle the existing piled up volumes completely. Scientifically, surely a nice attempt, but in practice it is just adding up to existing problems only.

...with further applications possible if it is made water-repellent, the research team said.

Sorry, you create a material that degrades easily in water, for the express purpose of being environmentally friendly, and now you intend to waterproof it?