I have had low-floaters that were dead. My first go-round in the incubator, I had a bunch of eggs die when they were late in development. They floated low, but didn't wiggle. After candling them to make sure, I opened them up (nervously) and . . . yup, all dead. If any of them had wiggled while in the water, that would have been a sign of life. Floating low, on the other hand, just tells you that they're late in development. And if an egg that's been incubating for 3 weeks floats high, it may be developing gas inside, so that's another thing floating can do for you: tell you which ones to get out of the incubator ASAP, before they blow!