From what I've gathered it's only considered dangerous if they're floating and can't sink to the bottom, in which case you have to lower the water level just to cover their skin which will de-stress the axolotl and hopefully allow it to recover. Some axolotls just tend to float on occasion, young juveniles seem to do it a fair bit; mine has a habit of doing it when it's nearly his feeding time but will always sink down to the bottom once he spots me.
There's lots more information on this thread.
Floating axolotl