Pedantic point: There's no such thing as 'duty free' for travel within the EU. There's only 'duty paid' (where you buy goods with the local rate of tax included).
Having got that out of the way, you'll probably find the best bargains on tobacco goods in the local shops in Bruges (or wherever you're actually going to - Zeebrugge is just a small village with a big commercial port). But the best bargains on booze (as long as you don't mind the restricted range available) will be at the ferry terminal on your return journey.
Don't expect really major discounts. Belgium used to be far cheaper than the UK for tobacco products but tax rises have brought their prices closer to those of the UK. Even so, you should find cheaper ciggies:
http://www.the-tma.or.../eu-cigarette-prices/
The special promotions on booze at the ferry port will probably be good value but your local Asda might still be cheaper on certain products.
Chris