If you can see how deep it is using the external cam, then it's not deep enough...:D
You can use the colours to have an idea (the standard ruler tool shows a chart next to it) Otherwise you can ping on arrival in the zone when you're still alone...

This is what I do too. But sometimes it's nice to see your boat floating 2m above the ground ;)

Remember, silent running isn't enough. It sets down the speed to 3 kts. That's way to loud. Put it on 1kts and they'll have a harder time finding you.