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.
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).
Source: work in the industry