> From a cave diving perspective, it would be useful to add local zero

depth lines using a similar process. Is there an appropriate line type

to use for this?

I have not yet drawn a wet cave, but I’ve been experimenting with

line border:presumed –clip off

for flood levels.  You need to use ‘adjust horizontal’ for the second line point to make sure the line stays horizontal after morphing.

The adding a bit of an area water inside a border:invisible line.

For a wet cave you might use area sump.

An example here

I have the flood levels in a separate map so I can easily toggle them on or off.

Bruce