I've worn a survival suit. They are reasonably well insulated, and proof against wind and water. Having got the excess water out of it, the remaining water will warm up and act as further insulation against losing heat through conduction. The clothing under the suit holds the water, so you don't lose heat by convection, and the suit is a rubberized fabric, so it doesn't lose heat by radiation.
Wearing it gives him a chance. Without it he would lose heat by all three methods, and probably die in a few hours.
Wearing it gives him a chance. Without it he would lose heat by all three methods, and probably die in a few hours.