First off out to 200 miles isn't international waters, those would be domestic waters in Canada, Europe or pretty much anywhere in the world - they refer to it as the Exclusive Economic Zone.
https://oceanexplorer.noaa.gov/facts/useez.html
Believe me it's monitored...
Absolutely mind boggling what satellites and aerial surveillance can do now a days, as well as port state control inspections, quality surveyors, class inspections, flag inspections and all of the other stuff that folks in the marine industry get to enjoy dealing with.
The surveillance is impressive and the fines are as well.
When you have a ship that generates 50 or 60,000 dollars a day in charter hire and carries half a billion dollars worth of cargo, the owners do not dare to take chances f'ing around with it.
Screw this up and have the ship arrested alongside for a couple of weeks, or banned from US, Canadian or European waters, or blacklisted by charterers makes trying to play it smart and save $25,000 in fuel costs by rolling over from HFO late start to look pretty dumb.
This isn't about environmental awareness it's about money...
That and the criminal side as well - the DPA (designated person ashore) as well as the Master and Chief Engineer being charged criminally is a pretty strong disincentive to cheat - It's not like gun crime in Canada - screw this up and you actually go to jail.