The best time to go for a cruise in Europe generally depends on the weather and the destination that you are going to. It is highly recommended that you go cruising in the months of fall and autumn when the weather is pleasant. Going on a cruise in the monsoons are to be avoided.
It is suggested that you wear comfotable and casual clothes while going on a cruise.
Every cruise has their own set of ticket cancellation and refund policies, so it depends on the cruise and the type of ticket that you have booked. Most cruises do not have a ticket cancellation/refund policy, while others do allow it if you visit their ticket counter.
Yes, most of the cruises go from one city to another. But, the sightseeing river cruises are just 1 hour long cruises that generally do not go from one city to another.
Yes, you need to carry passports, visas and other travel related documents while going for a river cruise in Europe.