Hello Stefan,
1) If you want to catch big cargo ships in Antwerp, drive up north to the Zandvlies/Berendrecht lock complex. Ships pass just in front of you. Great place to view ships. At Zeebrugge you can go to the Van Dammelock. There it's big car carriers most of the time (Wallenius Wilhelmsen, NYK, K-Line,...). You can also have a great view at the CHZ Containerterminal if you drive to the naval base and follow the route to the end. There is a little square where you can sit and watch the big container carriers.
2) For Antwerp: visit sss.be or loodswezen.be
For Zeebrugge: visit zedis.be
Cruise ships for Zeebrugge berth at quay 702 (there's a couple of them listed for arrival in September. Check the zedis site). You can view/take pictures when you're at the Van Dammelock. Cruise ships for Antwerp berth right at the heart of the city centre (near 't Steen). When you're in the city just walk to the side of the river Schelde (Scheldt) and look to your right or left to see if there are any cruise ships berthed. I don't have any info on sites containing info on cruise ships bound for Antwerp.
Hope this helps!
G.