Agüitar is a colloquial Mexican Spanish verb meaning 'to get sad' or 'to get down'. It comes from 'agua' (water), evoking the image of someone's mood getting rained on, and it's a playful, informal alternative to 'entristecerse'.
In the song, the line "No se me vaya a agüitar" ("Don't go getting bummed out") shows the singer reassuring someone not to let bad news dampen their spirits, keeping the tone light despite the subject.
Fuerza Regida and Junior H open "Whiskey Con Agua" already inside the car, windows up on a Mercedes: "Las ventanas del mercedes van arriba" [the Mercedes windows are up]. Two lines later the drink has taken over: "El alcohol nubla mi vista" [alcohol clouds my vision], and the song keeps piling on from there, a bump of cocaine ("Se amerita un pasesón de lavadita" [it calls for a fat bump of washed coke]) and bottles of champagne going off while "Las morras ya voltearon a mirar" [the girls already turned to look].
The title line, "Whiskey con agua, luces me brillan" [whiskey with water, lights shine on me], is really the whole point: everyone is watching. But there is a gun in the room the entire time, "Mi glock cortita dentro del morral" [my short Glock inside the bag], and respect here is something taken by force, not asked for: "Entre la mafia se va respetar" [within the mafia that will be respected]. Junior H and Fuerza Regida are not hiding any of it: the car, the drink, the drug and the gun all get named within the same eight lines, like a checklist for a good night.