Cleaning rivers and oceans is a huge task now.
A very rare few countries have laws for population control.
As far as I know, China is the sole and only frontrunner and leader in this.
Earlier, China was following a very strict one child law, and the law has been relaxed now allowing up to 2 children only.
All other countries need to have these laws perfectly in place with an iron hand, as huge populations bring problems of sewage and garbage dump of paper, wood, plastics, metals, industrial acid and chemicals.
1. Coming to the heading on how to clean rivers and oceans, we need to have sewage treatment plants and factories along all the major and minor rivers.
2. We need to have similar sewage treatment plants and factories along all the drains going into the rivers and streams.
3. A thick belt of plants, trees, and weeds need to be grown alongside all these drains and rivers. These have a huge capacity to absorb all the pollutants. In fact a certain variety of weeds and plants can easily thrive in these conditions, and reduce the foul smell, and we need to learn about them and allow them to grow alongside the banks.
4. Regarding garbage dumped in the oceans, we need to slowly and regularly pick them out in small and big ships, no matter what the cost. We need to put them up in landfills, reprocess whatever we can, and stop dumping it all back in the seas and oceans again and again with strictness and penalties.