Here is a modular system that can be used to develop scalable robots that can be deployed underwater. A team of MIT engineers used hollow structure voxels made up of cast plastic pieces. They are load bearing in one direction and soft in others.
This approach can be used to make eel-like or wing-shaped robots. As the researchers explain, their snake-like structure has 4 segments with 5 voxels each. It is covered with a waterproof skin. The above video shows it in action.