Illegal shipping is a small problem (~$100M?)

Illegal fishing is not - $10 - $23B annually in cost (2009)[1], destroys fish stocks, and contributes to slavery and human trafficking.[2]

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646833/

[2] https://fas.org/irp/nic/fishing.pdf

Illegal fishing is a big contender and is one avenue I think is sorely needed in this world of horrific natural/resource depletion and by-catch waste, despite what certain punters with an axe to grind (eg @mmjaa, below) may believe.

I would imagine the illegal shipping activity they are mainly targeting is the bypassing of sanctions. For example oil being transferred between tankers on the high seas and then shipped into North Korea.

https://www.reuters.com/article/us-northkorea-missiles-russi...

To establish precedent for the UK policing the worlds oceans - a goal its had on its roster for centuries now.