The ICP, International Certificate for Pleasure Craft, comes from KNMC or the Watersportverbond. The Dutch government lists it as proof of ownership on inland and coastal waters in countries including Belgium, France, Germany, Croatia and the UK, and notes that it isn't proof of nationality.
What a surveyor checks
The owner and boat named on the ICP against the seller and the boat. For nationality and the Dutch flag, the boat needs a zeebrief.
In the report
"ICP seen in the vendor's name, boat details matching. It proves ownership abroad only; it isn't a certificate of registry."