Negative space is essentially empty areas that don't contain anything. The idea behind negative space is that the empty area draws your attention to your subject. For example, if you have a white sheet of paper with a drawing of a small bee on it. The white paper draws your attention to the bee. The white doesn't directly contribute to the image but it draws your attention.
In your particular picture, the river is the negative space. I think what the person who offered the criticism is getting to is that the subject of the photo (I'm assuming the boats) isn't a strong enough subject to handle the negative space. If you recrop the picture so that only one boat is visible and only the river is visible (no banks, horizon, etc) then it would be a stronger composition for negative space.
You can read more about Negative Space from this article from Layers Magazine (Kelby): http://layersmagazine.com/negative-space.html