You unpack once. That is the whole argument, and for many people it is enough.
River ships are small — a hundred and fifty passengers rather than three thousand — they dock in the centre of town rather than at an industrial port, and the water is flat, which matters if you are prone to seasickness.
The Danube between Passau and Budapest is the classic. The Mekong from Siem Reap is the most interesting. The Brahmaputra is the one nobody expects and everybody talks about afterwards.



