When you use water on an egg near hatch - or an egg late to hatch, use fairly shallow water (couple of inches) at 99-100 degrees, as long as it's just deep enough to cover the egg BRIEFLY.
You place the egg in the water gently - making as little movement of the water as possible. Then watch. A LIVING chick will soon start MOVING around enough to move the egg and the water. Get it out fast - that one is alive! Yay. An egg that doesn't move at all and the water helps you TELL/SEE that clearly is a dead chick.
You'll get a few questionable ones, but it's actually a good way to tell when the eggs are WAY too dark to candle.
Just make it FAST - you are depriving them of oxygen, it's why they move.
Don't do this in front of children, this is a bad thing to let them think of doing when you are not watching. It goes on the list of what children should probably not see without gigantic lectures and a lock on your bator.