"I wouldn't mind seeing China if I could come back the same day" Philip Larkin
Cruising has become very big business in recent years, indeed you can now buy your own apartment in a cruise ship and spend the year afloat.
The market is probably divided between the small and relatively small 'special interest' cruises and the mammoth ocean liners. You take your pick.
http://www.hebridean.co.uk (as chartered by HRH for 2006 & 2010 family summer hols)
for a small family business specialising in luxury river cruises in Europe (and ocean cruising) see http://www.cruiseinrivercruises.co.uk
For a selection see www.pagemoy.com (or any week end travel supplement)
Cargo Ship cruises
a more unusual way to go - see Robin Smeeton's comments, who tried it.
for UK, try www.waterwaysholidays.com
for europe, try www.connoisseurafloat.com
but remember, a boat can feel very cramped if you take too literally the suggested number of occupants she can sleep - if there are 4 in your party, get a boat that sleeps 6 at least or you may feel very cosy. Also, hiring bicycles in France is recommended, to collect the croissants in the morning .