You will notice that there is a new ENI box available on the upload and edit pages. It's also shown in the "Identification" box on the photo page, if such a number exists/has been added for the vessel.
ENI is short for European Number of Identification, or European Vessel Identification Number.
This is a 8-digit number given to European inland vessels. If a vessel is given such a number, it must be printed on the sides and stern of the vessel. This number remains the same for the hull it's entire lifetime, independent of the vessel's current name or flag.
The system itself is inherited from Rheine vessels. Do not confuse this with the 7-digit IMO number!
Read more here about construction of the number, and country codes etc. http://en.wikipedia.org/wiki/ENI_number
Should you find any problems with this new feature, please let us know by submitting a bug report.