They have tiny glomeruli. They simply dry up, and remain that way until water returns. You are accidentally given a transfusion of distilled water, instead of blood. If this doesn't kill you, it will, at least, drastically reduce the concentration of dissolved substances in your blood. Ectotherms can generally be active over a wider range of temperatures than endotherms can.
Ectotherms can devote a larger portion of their energy to growth than endotherms can.
Ectotherms are more able to reduce their metabolism during times of low food availability. Ectotherms do not have a heat-related lower size limit. Ectotherms can generally survive better in resource-poor environments. Many of the cooling mechanisms that mammals employ involve increasing the metabolism. Increasing the metabolism can increase the heat of the body even more.