Is there any reflection map waterfall retexture existing?
Or do you know a tutorial(for noobs) how to implent reflections to a texture?
Not that I know, spotted http://pnmedia.gamespy.com/planetelderscrolls.gamespy.com/fms/images/mods/8225/1264948565_fullres.jpg the other day. From Part 3 (Canols) of http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=8225. Pretty spangly.
If you want to do your own then I'd have a shufti in the Construction Set forum and (maybe) at the NIFTools forum, it might be possible to bodge something together in NIFSkope.
I'd also really like to see better version of waterfall and water layers for fake rivers and lakes on grounds that are much above 0.
They're probably using Dongle's water meshes, I don't know of any others. I'm not aware of anyone doing anything to update them since their release.