Will cold water affect our throat / voice if we drink it before singing?
Also, in the long run (say if you drink cold water everyday), will cold water cause the throat / voice to be "lousy"?
Drinking cold water long-term won't make your voice "lousier". The whole idea behind singing is to have your vocal cords and surrounding muscles as loose as possible while singing. Hydration is a huge part of healthy vocal cords, so drink as much water as you can (hot, cold, or otherwise).
It's just recommended that if you get thirsty either just before or while singing that you drink room-temperature water.
The reason for this is because when muscles get cold they contract. Contracted muscles = tension. Therefore drinking cold water before or while singing causes your vocal cords to tense up and cause a "lousy" sound.