floating eggs

Old eggs float in water because of a larger air cell. (The air cell forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger.) To make sure the egg is not spoiled--break it into a clean bowl and check to make sure it doesn't have a bad odor or appearance. Eggs that are a week or two old can float in water however they are generally still usable.

try candling the eggs 

The water method will definitely not tell the difference between a live and dead egg. In fact in the live egg the air sac will actually grow faster than a dead or infertile egg.
Candling is definitely the way to go. There is a great sticky at the top of the page.

You can buy special candling torches but I still use an ordinary torch and a loo roll inner!

How long have they been in the incubator for?

Ok so at 23 days it'll be hard to see anything when you candle them, although you should hopefully be able to make out some movement depending on how thick the shells are.  It can take longer than 21 days for the chicks to start hatching, just make sure that the humidity is at the correct level and give them a few more days just in case.  Good luck.

At 23 days youcan hold them to your ear and tap the shell - you may be rewarded with a  tap back

Actually floating an egg at this stage of incubation can be indicative of live or dead eggs, but you have to be extremely careful about disturbing the temperature of the egg, and it isn't something I'd normally recommend