¡Aguas! is a warning of danger. You can specify the source of the danger with con.
- ¡Aguas! Watch out!
- Aguas con esa vieja. Watch out for that one (that woman). She’s a crafty one. She’ll trick you or get you into trouble.
¿Me echas aguas? is a way of asking someone to watch and make sure you don’t hit anything while backing up your car. The car watcher could then say ¡Échale! ¡Échale! to tell you to keep backing up, indicating the absence of any obstacles or dangers. Échame aguas could also be used more generally to ask for a lookout, for example, when you don’t want to get caught doing something prohibited.
Like ¡Aguas!, the interjection ¡Ojo! is used for getting your attention: ¡Ojo! ¡Mucho ojo! This is more specifically for reminding you to proceed with caution, to look at a situation carefully before deciding whether to get involved. It can be accompanied by a gesture where the extended index finger touches the face just below the eye. I’ve also seen ¡Ojo! on signs, letting customers know about a great deal.
Both ¡Aguas! and ¡Ojo! are somewhat informal. A more standard warning would be ¡Cuidado!