back up light

What is a good entry level secondary/back up light?

backup for what purpose, video, spoting, focus, signaling etc?

If its a very simple backup light for night dives, for example, a disposable battery (say 4x C cells) works perfectly well and they cost nothing. 

This is a great little torch:  http://www.inon.jp/products/le_light/le330h.html

Not super bright but good enough to get around, taking AA cells is a nice bonus as you can get them anywhere.