Assuming the water is warm enough that you don’t simply die of hypothermia, the next problem would be your skin. For reasons that still aren’t well understood, human skin starts to break down after continuous immersion in water of a few days. You’d suffer open sores and be liable to fungal and bacterial infections just from the spores on your skin, even if the water itself was perfectly sterile. The pressure of the water also reduces the circulation to your extremities and makes breathing more difficult.
The Chinese Government has been accused of immersing political prisoners up to the neck as a form of torture and victims are reportedly unable to stand or use any of their major muscles for several weeks after immersion of just a few days. Since this water is most certainly far from sterile, it’s hard to say how much of the effect is due to the water and how much is due to the contaminants.