>IANAO (I am not an Oceanographer) but would it make sense to use log scaling for the coloring of depth?

I've done a quick literature review (google + papers and text books in my office) and I can't find any examples of colouring the points on a T-S plot by depth, only by other *variables* e.g. dissolved oxygen or some other composition variable.

I think colouring by the depth is already somewhat just for the sake of it, and I think it would be best not to complicate by introducing log scaling.