I want to add let's say 11 water lines across the depth of my vessel using automation. How do I do that? and how to I get the height coordinate of those waterlines using automation?
Not all UI functionality is available in the Automation interface
there is not really a simple automation command to do that, its generally expected that if you have used automation to generate a model then you know its dimensions or you would have specific requirements for the waterline locations.
Otherwise you can scan all the surfaces in the model and work out their extents (from the control points) then use that height to determine where you want your waterlines to go