It kills in a frightening way: if a human r animal touches it, they are immediately pulled into the water and down. The victim cannot be saved; not even Sundered Ones can bring them back.
The only time it’s safe to touch black water is if it’s been separated from the main earth-wide body of water. Buckets are often used for this, though everyone tends to prefer making Sundered Ones fetch it – since they can touch it without harm, for reasons unknown.
Note: if you’ve read The Sundered, feel free to jump on over to Motherwater. If you haven’t read the book, don’t. You’ll regret it otherwise.