Like many of us, I rarely need to use a compass, but I do always have one underwater. I use a compass with hose mount, and keep it mounted on my back-up regulator hose near the second stage, which I keep clipped at my right lower BC ring. The mount allows me to twist the compass so it's in the right spot to look at when I look down at it At that spot, I can read it without having to hold or touch it. I've got enough going on with my housing and wouldn't want the clutter of a compass on it.
Hope this helps,