Due to a EU directive, from now on the Maritime and River Police will closely monitor the pleasure craft to and from third countries (=non-Schengen destinations).
This means that the crew of yachts departing from a Belgian harbour to a non-Schengen country (Great-Britain) are to report to the border crossing point of the Maritime and River Police in the port of origin. Pleasure craft coming from a non-Schengen country, have to call at a port that is a designated border crossing point and report to the Maritime and River Police based in said port.
In both instances, you need to hand over a document containing all technical characteristics of the vessel and the names of all persons aboard. A template for this document can be found in the Notices to Mariners nr 1(Annex 2) and can also be found on the nautibel website.
All marinas along the Belgian coast are recognized Schengen border crossing points.
More information in Notices to Mariners nr 1 and www.nautibel.be