I candle for all normal checks while hatching, but do water candling as a final check when eggs are way past due. Of course it's only for a few brief seconds and never if there is a crack or pip in the egg. Have never had any water enter an egg while doing it.
There is nothing better in this world than baby button quail. But hatching is so sensitive to Temps. Just slightly off and everything is delayed. I never give up on any egg until at least day 22. And I water candle every one before I throw them out. My very first hatch I ended up taking 3 eggs...