Yes, a wardrobe for narrow gentlemen might work. Alternatively, one like this (pull forward hanging space and shelves):
http://www.ebay.co.uk/itm/vintage-gentlemans-wardrobe-/321415090496?pt=UK_Antiques_AntiqueFurniture_SM&hash=item4ad5d51140
That particular one might be too tall, I didn't really look, but you get the point. I know you'd loose some hanging space by making the top less deep, and need to modify the rail, but you could do it, couldn't you? I'm not mad?
Usually, how wide is a narrowboat at it's narrowest point (that would be right near the ceiling, wouldn't it?).
Fair point Bottle, I wasn't meaning to write off the fitted aesthetic, I have seen some lovely ones, by this beholder's eye is not generally enamoured...