1) Do not scale the geometry, keep the original dimensions.

If you are just looking at the hull deformation caused by hydrostatic pressure, and not the deformation cause by heavy objects inside that are supported by the hull, then you don't need gravity.

2) Assume the submarine weight is in equilibrium with the buoyant force.

3) Assume a uniform pressure on all external faces => no net force.

4) "There are two choices to "ground" the structure so that Static Structural will solve without creating artifacts in the solution.

a) Use Symmetry. Slice the geometry with a vertical plane and eliminate half the geometry. Apply a Symmetry Boundary condition on all the cut faces (or edges) of the model. That takes away 3 DOF. Assuming the long axis of the submarine is parallel with the X axis and the vertical direction is the Y axis. Choose any vertex on the cut plane and add a Displacement BC of X=0 and Y=0 and leave all else Free. This point will have zero deformation. Choose another vertex a long way down the length of the submarine from the first BC and add a Displacement BC of Y=0 and leave all others Free.

b) If you don't use Symmetry and there are three fins, pick the same vertex point on each of the three fins. With those three vertices selected, create a Coordinate System. Change the type to Cylindrical. The new coordinate system should have the Z axis along the length of the submarine. Now each of those three vertices will get a Displacement BC with Y=0 and Z=0 and all others Free. The center of those three points will be the point of zero deformation. The points are free to move radially, which is the X axis in a cylindrical coordinate system.