Coma and death can result when a loss of body water exceeds what percentage of total body weight?

Between 15% and 25% of the body weight.

A 3% to 4% decrease in total body water, is mostly tolerated and doesn't cause adverse health effects.
A 5% five to 8% decrease can cause fatigue and dizziness.
Over 10%, can cause severe thirst, physical and mental deterioration.
Death or coma can occur when it's lost about 15 and 25 percent of the body water.