Cold blooded creatures do not have cold blood. Instead, their blood tends to match the temperature of their surroundings.
Warm blooded creatures have to burn fuel constantly to keep their temperature constant. Cold blooded creatures don't and thus can get by with less food.
Water Monitors are more active than reptiles of their size because they are able to maintain an almost constant body temperature. They do this by choosing appropriate micro-climates in their habitat; hiding when it's hot and in warm places when it's cool at night.