If you are in Mexico and you hear ¡aguas! don't look for water, look for something that can hurt or harm you, and probabilities are that it's in close proximity!
¡Aguas! is used as "Cuidado" in a more colloquial way.
¡Aguas con el escalón! Careful with the step!
¡Aguas con los coches! Careful with the cars!
¡Aguas, casi te caes! Careful, you almost fell!
Aguas con lo que dices enfrente de esa persona. - Careful with what you say in front of that person.
Now, if somebody asks you: "échame aguas", they are not asking you to pour water on them, they are asking you to be on the lookout. Here are some examples:
1. Juan, quiero esconder este regalo para Jorge, párate junto a la puerta y échame aguas.
Juan, I want to hide this gift for Jorge, stand by the door and be on the lookout.
2. No sé si quepo en este lugar, échame aguas para que me estacione bien y no le pegue a los otros coches.
I don't know if I fit in this spot, keep an eye so I can park properly and not hit the other cars.
Some people believe that "aguas" used in this manner comes from the time when people used to empty their buckets of water onto the streets (from above) and you had to be careful not to get wet. Whether that's true or not I can't say, but if you hear ¡Aguas! be careful