Amsterdam's official tourism website is here:
http://www.holland.com/amsterdam/gb/
The weather is similar to London.
The currency is the Euro.
There's the same wide range of food available as in any other major international city. Menus often have English translations (and English is widely spoken). Dutch speciality dishes are often based on pork or fish. You'll also find loads of superb fattening pastries!
Most caf�s have the same rules of conduct as in the UK. I suspect that what you're referring to in your question is the 'coffee shops'. If so, see here:
http://www.amsterdam.info/coffeeshops/
I assume that you're travelling from Newcastle (because,as far as I know, that's the only departure point for mini-cruises to Amsterdam). If so, your vessel will either be the King of Scandinavia or the Queen of Scandinavia. Nobody really cares what you wear when dining. Certainly, if you go for the cheapest (self-service) option, you'll see as many different types of dress as you would walking down any High Street (i.e. from 'very smart' to 'downright scruffy'). However, some people might choose to look smart when dining in the a la carte restaurants. See here for details of the on-board catering options:
http://www.dfds.co.uk/DSW/EN/OnBoard/na/Dining .htm
Chris