Control Torque Inputvector

Question

I'm faced with the following problem:

I'm trying to implement a rigidbody-based movement for my submarine game which means I want (need?) to useAddForce() and AddTorque() in order to get proper collision detection.

Currently I'm achieving the desired rotational behavior by quaternion multiplication/rotation.

But with this approach, the submarine bugs through the terrain. To avoid that it seems my only option is to use a rigidbody.

Here's a video illustrating the problem.

On the left is the correct behavior. I want to keep the nose pointing where it has been tilted at and not rotate around a fixed axis, Vector3.up in this case.

How can I calculate the correct input for AddTorque(vec3) to get the result I need?

Left attempt with quaternions (what I need):

input2 = Input.GetAxis("Sideways");
transform.rotation =
    Quaternion.Euler(0, input2 * Time.deltaTime * rotation_speed, 0) * transform.rotation;

Right attempt with torque (which doesn't quite work yet):

// in Update()
rotateY = Input.GetAxis("Sideways");

void FixedUpdate() {
    // experimenting with AddTorque() and various inputs didn't
    // get me any further
    rb.AddRelativeTorque(Vector3.up * rotateY * speed);
}

I hope someone can point me in the right direction, thanks in advance.