To see depth maps, it’s required that you have a Fishbrain Pro account. You should then be able to view them from the bottom navigation of the app, and you should by default see depth maps.

If depth contours are not shown on the map, try tapping on the cogwheel on the sidebar navigation of the map and see if you need to turn depth maps on.

If you still don't see depth maps, it could be that the area you are viewing lacks coverage.