I added a node and tags for this object as seamark. You cannot set seamark:type=beacon_special_purpose on a polygon, this is why I added an extra node, it will only appear on openseamap and the like.  The name "Het Baken" is written on the sign at the cycle path leading there. 

I just looked into the data again. I just missed that there already was a single node for it. I fixed that.