We had so much fun having a water fight with water-balloons in the backyard.

water-balloons 是复合名词，由 water（水）和 balloons（气球）组成。 water 指"水"，balloons 是 balloon 的复数形式，指"气球"。 water-balloons 指"水气球"，即装满水的气球，常用于打水仗游戏。