翻译:水往低处流,是个不可改变的自然原则

It's an unchangeable nature principle that water flows downwards.
参考：
Man struggles upwards; water flows downwards.

一定要用law of nature的话
It's an unchangeable law of nature that water flows downwards.

Water flows from high position to low position is an unchangeable natural principle.

It's an unchangeable natural that water flows from high position to low position.