Autodesk Navisworks

[#1]

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).

[#2]

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.