Having recently been testing my submarine, hundreds of times, I think this is a great idea. I would like to sea an indicator for the Center Of Buoyancy and would even go a step further and suggest a value be displayed in the engineer's report in much the same way as mass; perhaps negative numbers could be you will sink, zero meaning neutrally buoyant and positive numbers you will float.
On a bit more specific note, now we have a better buoyancy model I would like to see stock parts that allow you to pump water in and out of ballast tanks to raise and lower your vessel. I am aware this is not a part that would be used widely among the community (but may be with a scientific incentive in career mode), and I apologize if this is the wrong place to post suggestions. I'm not the kind of person who usually posts on forums.