Water hags can be found in swamps and shallow water.
They are weak against Igni.
As creepy as they are dangerous, hags are rather horrible creatures that pose a threat both at close range and from afar. When you're at a distance, water hags will throw globs of mud at you, which will obscure parts of the screen if it hits. Not only that, but it'll also stun you for a short time, so it's always important to sidestep this particular attack.
When they get in close, hags make use of their sharp claws, swinging wildly at you. If you're lucky, you'll be able to block the first hit and then roll away, but let a hag continue its combo, and it'll break your guard and stun you. You also need to be careful of their ability to disappear and reappear in water. If they get the chance, they'll usually emerge behind you, so as always, be ready to roll.
Grave hags may be considered a bit more horrific, though, thanks to their whipping tongue attack. Using their elongated muscle, they'll lash out at anything within a certain distance, and it's got a very wide radius. However, a quick back step is usually enough to get out of the danger zone.
Arguably the best way to deal with either creature is to make good use of quen so that you can weather the first blow before moving in for the kill with several sword swings. Hags tend to stagger quite easily, so once you're in striking distance, don't stop unless you know that the fiend's going to retaliate.