An immatriculation is an identity document for pleasure craft that proves that your boat has been added to the pleasure craft registry. That also gives you the right to sail under the Belgian flag.
This document is mandatory when sailing in the high seas, foreign waters, the Belgian territorial sea, the coastal harbours, Ghent harbour, the channels Oostende - Brugge, the part of the Ghent-Terneuzen channel under Belgian jurisdiction and the Lower Scheldt.
An immatriculation is necessary for pleasure craft with a length from 2,5 metres up to 24 metres. This means that kayaks, gondolas, canoes, pedaloes, surf boards or similar objects that measure less than 2,5 metres are exempted from the need for an immatriculation as are pleasure craft measuring over 24 metres.
The exemption also applies to pleasure craft used for transporting more than 12 paying passengers or used for fishing at sea with paying passengers on board. In that case you do need to have a certificate of registration.