No magnet fishing in Vlaanderen without your Erkend Metaaldetectorist badge. Get it here:
More archeological rules and laws, some quite interesting, especially the Geoportaal and rules for ‘Accidental Finds’
If you want to go magnet fishing in public waters in Flanders, chances are very likely you are fishing in waters governed by the Vlaamse Waterweg. Here’s a link to their policy concerning magnet fishing.