For most indoor plants or seedlings, room temperature water is best for the plant's health. Keep a can filled for indoor houseplant watering. Let the water stand in the can. This also allows fluoride to evaporate, as the chemical can damage some plants.
Never allow plants to wilt. Use room temperature water for the most successful growth experience with your plants, but cold water is better than no water if the plant is thirsty. Most outdoor-growing fruits, vegetables and flowers take cold water with no side effects. Make sure to drain any hot water from a hose that's been left in the sun. Hot water can shock any plant.
Plants growing in ponds or other bodies of water react strongly to water temperature changes. Some aquatic plants thrive only at levels in the pond where the temperature is optimum for their growth.