This is the 4586th most frequent Chinese word.
水上
"On the water" or "aquatic."
In this sentence, '水上' ('on the water') refers to the surface of the water, indicating activities performed there, like playing.
孩子们在水上玩得很开心。
The children are playing happily on the water.
Here, '水上' means 'above the water' or floating or built atop the water surface.
他们在水上搭建了一个浮桥。
They built a floating bridge on the water.
In this sentence, '水上' serves as an attributive modifier, indicating activities related to water surfaces, i.e., '水上运动' ('water sports').