Essentially you'll die of acute hyponatremia, low blood sodium.
Some chronic symptoms:
Headache, nausea, vomiting, muscle cramps, lethargy, weakness, restlessness, disorientation, ataxia.
Some acute symptoms:
Seizures, coma, permanent brain damage, respiratory arrest, brain stem herniation, death.
So it's probably pretty painful and uncomfortable, muscle cramps can be exceptionally painful towards the higher end of the pain scale, and this is assuming you can force yourself to drink enough water, which is one of the issues. The main issue if you do drink enough might be how long it takes to die this way, you'll probably suffer with at least a headache and vomiting for a long time, it causes cerebral edema i.e. it causes brain swelling, so it might feel like your skull is being crushed in a vice. Also if you don't die, you might be left in a coma and/or with permanent brain damage.