Negative space is the part of the image that does not contain the main subject. It’s often an empty or visually quiet area, and its job is usually to draw attention to the subject by giving the eye fewer other places to go.

It works best when that surrounding area is simple or nearly empty. For example, a small subject against a large plain background can feel strong because the empty area emphasizes the subject.

When someone says there is "too much negative space," they usually mean the non-subject area is overpowering the subject rather than supporting it. In your example, the river/background may be acting as negative space, but if the boats or other subject aren’t strong enough, the empty area can make the frame feel unbalanced or lead the eye away.

So yes: negative space is generally any area without the subject or point of interest, especially if it is visually simple. Whether it helps or hurts depends on whether it reinforces the subject or overwhelms it.