They are also surrounded by fluid that as ballast affects the density/volume/buoyancy/displacement(?) of the hull (and the inertia of the craft), which could be

They are also surrounded by fluid that as ballast affects the density/volume/buoyancy/displacement(?) of the hull (and the inertia of the craft), which could be steel-plated [3d printed and/or pressure molded] biocomposite that lasts for spec years in seawater.