I was afraid the Editor method was the only way to go. I was hoping that perhaps the depth info was 'hidden' somewhere in the game files and it would just be a matter of 'labelling' the map.
I haven't touched the Editor as I found the SHIII one very convoluted and intimidating. But I'll have a go as you suggest and post my findings. It's the least I can contribute given the amount of work the real modders do!!