Ok so you know how Death Knight's Death Grip doesn't exactly work in water right?
WRONG!
If you want to have your Death Grip work clearly unintentionally, simply swim all the floor of any body of water and it will work. Simple as that. May or may not help you out, but it sure helps me out.
On a further note:
Just swim all the way to the bottom almost as if you plan on walking on the bottom. However, it's obvious you can't. Just do it and use Death Grip on a mob in range. Viola!