We prove uniqueness of smooth isometric immersions within the class of negatively curved corrugated two-dimensional immersions embedded into
R
3
\mathbb {R}^3
. The main tool we use is the relative entropy method employed in the setting of differential geometry for the Gauss-Codazzi system. The result allows us to compare also two solutions to the Gauss-Codazzi system that correspond to a smooth and a
C
1
,
1
C^{1,1}
isometric immersion of not necessarily the same metric and prove continuous dependence of their second fundamental forms in terms of the metric and initial data in
L
2
L^2
.