Throwing cold water on someone's ideas or plans means to discourage or dampen their enthusiasm. It's a metaphorical expression that implies pouring down frigid water to put out a fire of passion or excitement. However, there are several antonyms that can be used in its place to convey the opposite message of fostering positive energy and encouragement. You could say "provide a warm welcome," "give a boost of confidence," "offer constructive criticism," "give support," "lend a hand," "be a cheerleader," "encourage," "empower," "motivate," or "inspire." These words and phrases imply something dynamic, inspiring, and positive, rather than the chill and negativity of throwing cold water on something.