相当于enough,
as 一般修饰原级,表示比较的意思,much表示量的多少,修饰不可数名词（my height is not as tall as you 我没你高）,所以此处可理解为一个省略,更容易消化：还原：you don't have as much water as （that）is needed in your system.与你系统中所需的水分量相比的话,你没有足够的水分.that可省略,带指 水量.
引申翻译为：你的水分不足以满足系统中的水分需求量.
as much是定语,用来修饰water.