To further demonstrate the potential of our data-driven approach, we tested our training model in an underwater environment. A manta ray-shaped prototype was fabricated using the same settings of FBG fiber, following a similar procedure with the previous A4-sized design (Figure. 4a). An optical fiber with 29 FBGs was adhered roughly along the edges of the prototype (Figure. 5a), and its FE model is shown in Figure. 5b, where the red nodes represent the location of 8 EM tracking coils for the model training (ground truth). Upon vertical actuation (1 Hz), the manta ray was displaced vertically underwater. The sensing performance of the manta ray fins can be referred to in Video 3 as well as Figure. 5, where Figure. 5c shows the ray at three typical deformation instances and Figure. 5d illustrates their corresponding real-time shape reconstruction. The ever-changing water drag did not hinder the sensing performance even with the model training and data-driven analysis initially conducted in the air.