The Sentinel-2 mission is doing that [0]:

> Systematic global coverage of land surfaces from 56° S to 84° N, coastal waters, and all of the Mediterranean Sea

The constellation is not complete yet though. Sentinel-2C is planned to launch in September [1].

[0] https://en.wikipedia.org/wiki/Sentinel-2 [1] https://www.satnow.com/launch-mission-details/sentinel-2c

Additionally, the only sea covered in its entirety is the Mediterranean. Generally, constellations don't do captures over open ocean as researchers/customers tend to be much more interested in events on land; this makes it difficult to do long-term analyses of marine events as the data just simply isn't captured.

Source: work in the industry

True, but coastlines are well covered. Assuming the pollution comes from the coast it should be fairly easy to determine what the hotspots are (see Po river on the map in the article).

> I'd assume it's also a long way to go from doing it for a small section of the world to doing it everywhere

This probably isn't a good assumption. It's likely more about it being much faster to iterate/validate the methodology on the smaller dataset of just the Mediterranean (2.5 million km^2) before spending the effort to run it on the entire ocean (361 million km^2, 144x larger data).