In theory, you can use the depth highlight layer anywhere that you have isobaths (depth polygons).
The depth highlight layer works by simply creating another copy of all depth areas in a single colour, located underneath the existing depth areas. The reasoning behind this is that chart categories cannot be disabled by default when creating an AT5 map, so placing the highlights behind the map means that the map is shown in the normal state by default. To highlight a depth range, the depth area category for the required range is deselected. This lets the highlight "shine through". Although this set up works very well for standard vector maps, where isobaths cover the entire water body, there is one obvious limitation: The depth highlight layer doesn't work well with transparent isobaths. In this case, the depth highlight layer would be visible through the transparent isobaths by default. You can get around this by deselecting the entire depth highlight category, and re-enabling only the levels that you want, but it is a bit more work to do on the water.