The T in water is called a flapped T. For Americans and Canadians, T is sometimes pronounced as an aspirated T [tʰ], unaspirated T [t], unreleased T [t̚], or a short, soft d-like flapped T [ɾ]. The flapped T is not a [d], although to non-native speakers, it sounds like [d]. Rachel's English provides excellent pronunciation videos on youtube. She has a phonetics background and gives accurate information. Here is one of her flapped T videos. https://www.youtube.com/watch?v=mUupfHhOnHg I find that she overpronounces the flapped T as a full [d]. This may be deliberate to demonstrate. And here is more information about the several allophones of /t/. https://www.italki.com/discussion/213450

Hi Shami, what you are probably hearing is "D" Wader, if you do a Youtube search for differences between UK English and US English you will encounter many videos explaining how the pronunciation varies between the two Countries... As a British person I would pronounce the "T" in Water. Supper holly and English with Lucy would be a good starting point.

It's just that it might sound like that when spoken quickly. They normally pronounce the "t" like a "d".