Although it's an option within the search box, I don't understand how the shipspotting system could correctly pull vessels out by flag, unless it's using the old Grosstonnage.com data. How would it know what the flag was when the photo was taken? Ships have many flags throughout a lifespan, so even if it is using the Grosstonnage data, it's going to generate their last known flag.
Brgds
Phil