The phrase "threw cold water on" is often used to describe the act of discouraging or dampening enthusiasm for something. Its antonyms could include terms like "encourage," "motivate," or "inspire," which all convey a positive and energizing effect. Other antonyms for "threw cold water on" might include "sparked," "ignited," or "fanned the flames of," which indicate that the opposite of the phrase is to stoke enthusiasm and passion rather than dampen it. "Nourish," "foster," and "cultivate" could also be considered antonyms since they carry similar connotations of growth and development.