I am working with a model of an offshore platform.
I'd like to show the Sea level (part way up the legs).
Can I alter the horizon elevation?
Can I add a plane to represent the Sea level? (which I can preferable go below and above).
The way I would do this is create a new AutoCAD drawing,
draw a circle at 0,0,0 with a diameter of a mile or a km, and then extrude it 1 inch or 1 cm down.
Now you got a new DWG that has the plane
Append this DWG to your model.
"File units and transform" this model or all other models until it is at the right spot.