Labeling Rousseau naive is offensive. By simplifying, Rousseau invents a language of abstraction similar I think to Gauguin's, but more radical. That this is deliberate is clear from his sketches and studies.
The study for Family Fishing is proficient, "painterly", and naturalistic. The river curves, water moves, trees rustle. Its perspective is reasonable. The finished work has become geometrical, the riverside and bend are hyper-precise straight lines with right angle, a row of trees stands unnaturally like soldiers at attention, the water is still, the trees are still, the perspective is flattened past reasonable reality. This is skillful production of representational language. It's principled and under control.
I find his most extreme abstraction in his portraits, where faces are rigid, mask-like, formalized. They're both simplified and formulaic, lacking individualization. In today's language they're anti-subjectivized, a concept unavailable to his contemporaries, whose interpretations fall back on naivety as one expression of this absence.