Underwater your instruments whether it's your SPG, Depth Gauge or Compass need to be reliable, clear and easy to read.  If you are diving at night, then be sure they light up.  

Getting lost is never fun and wastes time, so be sure you have an easy to use compass.  Practise on land with it and remember to set a bearing to your exit at the start of each dive.  

Even if you are using a dive computer that is air integrated, we believe you should still carry a compact SPG as a back up.

Remember you need to be able to attach your gauges so they can be easily found, but so that they are not dragging along the bottom.