composition - What is a negative space in a photograph?

From here: http://1x.com/forum/photo-critique/31871

you have far too much negative space so the eye goes toward the back...of the picture, and there is not much to see there...either.

What qualifies to be a negative space in a photograph? Something which doesn't have a POI?

Answer

Negative space is the part of the image that does not contain your subject. It has the strongest effect when it is literally empty as some of my images below, as they draw attention to your subject, basically by not giving your eye any other place to rest!

In your image, while I wouldn't have thought of it as negative space, the colorful boats do command attention against the backdrop of the river. It would be a very different picture without them - unavoidable as you say, but maybe try cloning them out.