If a drop of water hits something at bullet speed, will it damage it? I can't find this anywhere. If a drop of water hits a glass for example, at the speed of a bullet, will it damage it? Or will it just make the same as a raindrop?
 A: Wet steam erosion is a big problem in steam turbine design, as you can see from this image:

So yes, water droplets with enough energy can certainly cause damage to even objects made of steel. With enough energy, it could certainly cause damage (or destroy) your glass.
Water jet cutting is another good example of high-energy water being used to damage (cut) something:

A: It depends on a lot of things.
How big is the drop, and how fast is it moving, really? Most raindrops are smaller and less dense than most bullets, so in general a water drop moving at the same velocity as a bullet will carry less kinetic energy. Since a water droplet isn't a hard object, that energy also will be distributed over a longer interval than a bullet impact. So, it would likely be less damaging than the bullet.
Will it "damage" your glass? That depends on the glass. After all, some glass resists bullets.
A: I think the drop of water is too light to cause a damage to your front window of a car. Since, the drop is not rigid, distortion occurs in itself causes a vary quick reformation in the spatial arrangements of the molecules and can break subsequent hydrogen bonding of water, which is more unlikely in the causualty of damages. Moreover water can beswept out easily due to the less adhesive property and small coefficient of restitution. Thus apart from the formation of ice crystals, it can't damage a hard glass, but may break a weak one, depending upon the nature of the glass concerned.
