Sorry but I did not get a chance to post my opinion.
Very sad indeed, about the woman died after she drank too much water too fast in the California contest.
To give you an idea of what likely happened in this case. When you drink too much water like what she must have done it is possible to get water intoxication which causes among other things swelling in the cells as well as an imbalance of the bodies electrolytes. The swelling of the cells causes increased pressure on the brain cells, heart etc.
As far as not going to the bathroom well that in it self would not kill someone but can lead to bladder or UTI's, (Urinary tract infections). It is not likely that toxins would build up as the Kidneys will filter very well, in a normally healthy person. Actually your ability to hold your bladder full to the point of harm, other than possible infections, is not likely. What happens is that you get urges to pee in intervals that get stronger. There are 2 sets of muscles for the bladder. The first set of muscle is the "internal urethral sphincter" which we cannot control and when it decides to open we then rely on the "external urethral sphincter" which is the hold it muscle we control and use until we get to a bathroom. When the need to pee is strong and is present a long time the pressure on the external muscle is great and thus it becomes tired. Like any muscle in your body the more you use it the faster it gets tired. Many can hold it for some time but sooner or later that muscle will become so tired that you start to lose feeling and will start to leak to the point of wetting your pants. So therefore it is most unlikely that anyone can harm themselves by holding their pee too long, except infection, as no matter how hard you try you will pee one way or another.
The only way to do this is to do it gradually and even though it is not suggested anyone do this drinking water over a long period of time to test ones endurance and to see who runs to the bathroom first. Too much water in short periods of time is not good at all.
My suggestion if you must try this type of thing do it slowly and make sure that as you reach the bursting point you are close to a bathroom cause once you pass that point where you cannot hold it in you will be soaked if not careful.