Just get the ferry they don't have x-ray things in either Rotterdam Europort or Ijmouden (Amsterdam) port, I've been over six times in the last 12 months and have only seen a sniffer dog once, I also heard that they (the police and dogs) are only looking for large amounts of currency, as there is a problem with money laundering. Get the ferry from Hull, it's a good night out on the ferry then you can get a coach to either Amsterdam or Rotterdam. (I would just like to take this opportunity to say a have never smuggled illegal substances and never will :hippy