I don't think UN is sounding the alarm of the planet running out of water all of a sudden, even they understand we have huge oceans that aren't going anywhere.

The report itself (https://collections.unu.edu/eserv/UNU:10445/Global_Water_Ban...) does actually talk about a lot of the background, why's and how we can start addressing it, in a very fleshed out form + an executive summary at page 13.