If the thing being saved/conserved is a single object that is consumed all at once, or money, then save is typically the appropriate term. If you have a bunch of X and are trying to use as few as possible to make sure you have more X for later, conserve describes it, but so does save. Hope this helps a bit 🍀☺️

"We need to conserve" means we need to establish better habits regarding our water usage. It means we have to be more careful not to waste water. "We need to save" can mean the same, but it implies more of an immediate need. It suggests that we need to start stockpiling water so we have some extra supply on hand.

Conserve is a more academic word. It also has more of a sense of 'looking after' or 'managing' the water, rather than just not using as much.

It is more natural for a native speaker to say, "save water", rather than to say "conserve water". Pardon me for not going go into the details of the meanings, they both mean the same and I wanted to point out which one is a more natural way of speaking it :)

我们需要省水 we need to save water