Travel to Brussels by Ferry
If you really want to travel here by sea to Brussels, then the easiest ways are from the United Kingdom by ferry disembarking on the coastal cities of Bruges or Rotterdam. From the coast take a train from Bruges or (Brugge) Central station or Rotterdam Centraal to Brussels-Midi. Trains run 7 days a week on an hourly basis.
P&O Ferries has connections from Hull to Bruges, anchoring at Zeebrugge port, terminal Leopold II Dam (Havendam); and also from Hull to Rotterdam port. From Rotterdam and Bruges there are trains every hour to Brussels. All ferries are overnight and depart at 19:00 to Bruges and 21:00 to Rotterdam in specific evenings. Check the website for more details before booking.
Another alternative is to travel from Ramsgate port (near Kent) to Ostend and then take the hourly train to Brussels-Midi station.
Operating a route from Dover (UK) to Calais, drivers can reach Brussels from Calais (France). They can cross the English Channel by loading their car onto a ferry operated by Sea France and then driving towards the city.
This ferry travels from Rosyth (Scotland) to Bruges and also from Dover to Dunkirk (France). From Dunkirk the best way to travel to Brussels is by driving or by train (changing at Lille).
There is a river cruise ferry the River Cruiser m.s. Victor Hugo which sails from Amsterdam to Brussels from Newcastle operated by Scandinavian Seaways. The cruise is a part of a journey from Newcastle and there is no indication of a stand-alone journey from Amsterdam to Brussels.