the boiling water in my kettle gushing ... and slowly but surely, evaporating and disappearing into the mist.

As WW says, there is a missing link in the original metaphor between 'evaporation' and 'mist.

"evaporating and disappearing" into mist nicely fills the void.

Thank you, maahey, for supplying the missing link.

BTW, is "evaporation into mist" any worse than "clouded in a sea" ... as in the following example of a mixed metaphor (from UVic Writer's Guide).

MIXED METAPHOR

A mixed metaphor attempts to create an extended comparison but fails because it is not consistent with itself. For example, in an essay on the language used in describing pain relief medicine, a student wrote:

"The topic of pain relievers seems clouded in a sea of medical terminology."

The metaphor is mixed because the images of cloud and sea do not match. The student should have said either "drowned in a sea of medical terminology" or "clouded in a fog of medical terminology."

Metaphor can be effective, but do not put too much weight on your own ingenuity; it might collapse under the strain.