你混淆了不可数名词和单数名词。
《牛津》supply
[countable] an amount of something that is provided or available to be used 具体提供的东西
[uncountable] the act of supplying something; the fact of receiving something that is being supplied 提供这个动作或接受被提供的东西这个接受动作
The water supply is unsafe.供水不稳定。这里指供水这个动作、这件事,为不可数名词(抽象名词)。
Water supplies to the house are at low pressure. 这房子的供水水压低。这里指供应到这个房子不同水管处的水量,为具体名词,不同水管的供水量,所以用复数。