With CAH, potassium usually rises as a result of sodium depletion (salt loss) and can be a sign of of adrenal crisis.  The problem here is that once it starts rising, it will keep rising as long as the sodium falls.  In situations like this, the child should probably be hooked up to an IV of electrolytes and glucose and should definitely be given Solu-cortef.  While water may help, theoretically, to "dilute" the blood, you also run the risk of depleting sodium levels even further.  If anything, a saline solution should be given (never just plain water).  Solu-cortef is great in that it also has some mineralcorticoid properties and is the fastest acting.

The sodium-potassium balance in the body is EXTREMELY important. When levels go out of sync, blood-pressure drops, shock can set in, and the heart can experience fibrillation and arhythmia = very bad news.

In addition to sodium and potassium, renin levels should also be checked.