Learn Hardcore Spanish (Spain): Es mejor no quedarse sin agua en la montaña. - It is better not to run out of water on the mountain.

Breakdown of Es mejor no quedarse sin agua en la montaña.

Why is it quedarse sin and not just quedar sin?

Both exist, but they’re used a bit differently.

Quedarse sin + noun is the most common, everyday way to say “to run out of / end up without.” It often highlights that the subject is affected by the loss: Nos quedamos sin agua.
Quedar sin + noun also exists, especially with inanimate subjects or in a more formal/newsy style: Varios barrios quedaron sin agua. If in doubt, use quedarse sin, which is always natural in this meaning.

Could I say estar sin agua instead of quedarse sin agua?

They’re different:

Estar sin agua = to be without water (a state): Estamos sin agua (we don’t have water).
Quedarse sin agua = to run out of water (a change/result): Nos quedamos sin agua (we used it up / it’s gone). In your sentence, the idea is “don’t end up running out,” so quedarse sin fits best.

Where does no go with an infinitive and a reflexive verb? Why no quedarse and not no se quedar?