Water on the left captures the meaning literally: liquid pouring out fast. Whether of rivers, rain or - bluntly - diarrhoea, the action is a sudden outflow of fluid, putting 泻 with 流, 漏 and 灌 in the spillage family.
Carries the sound, with a small tonal shift from xiě to xiè. After PRC simplification the right side reduced from traditional 寫 to 写, but the phonetic role stays intact. The write meaning is unrelated to outpouring.
