Haven't done this as a ctb method but have had it otherwise for "normal" reasons (it's not really a normal condition to experience so all reasons are kinda weird) and i can confidently say its a wildly awful time. Full body nerve pain, some vomiting, you might piss yourself (your body very badly wants to be rid of all that water) and its actually pretty unlikely to kill you. I was in constant pain for about 3 days and if you change your mind at any point the treatment is wildly unpleasant.
Water intoxication dilutes the sodium levels in your blood so dramatically that you cannot function, but you cannot solve that like you normally would, with electrolyte drinks and such, because that will also introduce more liquids. So the solution is consuming pure salt. Like, a lot of it. If you're trying to hide an attempt (or in my case at the time trying to hide something dumb you did) that will mean you're eating enough pure salt for your tongue to bleed before it gets you back to almost normal. Not sure if it did any significant long term nerve damage for me but it definitely can.