I am building a world that used to be almost entirely covered in water but was struck by a large metallic asteroid (roughly the same mass as the one that killed off the dinosaurs) which boiled off most of the water leaving only a few small oceans behind. My question is if that is based in reality. Would the evaporated water be ejected away from the planet or would it simply evaporate and precipitate later resulting in a minimal loss of water? What would be some other long term effects of this disaster?
The short answer is No The slightly longer answer is, Not in the way you'd like to imagine it
If we wanted to boil off the seas, you need a lot of energy: your asteroid would need to be large, rather the size of a small moon than a dinosaur-killer. Even a swarm of smaller asteroids would be less than likely to manage that while keeping the crust even remotely intact. It's more likely the crust would be pulverized and the planets inner heat itself would boil off the water.
On a planet where the sea-bed was fjord- or funnel-shaped a collision from the right direction might cause a wave to get concentrated and spit water into space. However the collision would need to be massive: we're still talking small-moon-sized asteroids. For earth, the escape velocity is 11km/s or 33-35 times the speed of sound! The wave-pressure alone would probably be enough to shred large areas of the ocean floor crust. As the remaining water would pool back into a large "puddle" it would probably meet open lava all over the place.
There's no likely scenario where a simple impact could cause a runaway greenhouse effect either, except for very rare vulanic varieties (such as a giant pocket of carbon dioxide and sulfur-hexafloride). If your planet got ridiculously high amounts of atmospheric oxygen, you might trigger conversion to ozone, which also is a greenhouse gas.
:Edit: One option I forgot to mention was large clusters of methane hydrate. Also known as "methane clathrate" this compond of 4 CH4 and 23 H2O builds at low temperatures and high pressures at sea floors, contains 13.4% methane by mass, remains frozen up to -2°C at 1 atmosphere and decomposes above that. A direct hit might cause an underwater-mountain-range of it to decompose, foaming up the water, reducing its density, and furthering the decomposition. :End-Edit:
If your planet has a large sub-sealevel area that is stuffed with radioactive isotopes, probably of vulcanic origin, a large hit might break some natural dam, and drain the seas into it. The water would act as a moderator and boil off, powered by the massive natural fission plant that just happened. It might even shoot up in large steam-explosions, simmilar to a gysir. Those probably still wouldn't make orbit though. Water-vapor is a potent greenhouse-gas on its own though, so it might be just enough to trip a runaway greenhouse effect.
Frankly enough, the opposite route might be easier: Think of a dense atmosphere keeping the planet warm. If your asteroid (-swarm) pollute's it enough to cool down a bit, there might be a runaway icehouse effect: the greenhouse-gasses raining down or being reacted away cools off the atmosphere, which in turn rains off more greenhouse-gasses. As this reduces atmospheric pressure, water may evaporate (or freeze and be "unavailable"). Also the thinner atmosphere makes it easier for solar wind to carry away lighter gasses - like water vapor. This might be made easier if the impact somehow collapsed a previously active planetary magnetic field as well, but even then, this would be a rather slow process. Still this is probably the best chance to achieve that "water-poor, post-asteroid, post-sea-world" planet.
Evaporating the water is not enough to take it away from the planet, you would need to impart it also enough velocity to escape the gravity well, meaning at least 11 km/s.
It might be easier if you give enough energy to dissociate it into hydrogen and oxygen, with the hydrogen which can more easily escape as it is not trapped even at our current temperatures. However, splitting water requires also energy, not mentioning that the loss would happen over geological times, thus re-oxidation would happen.
Much easier is to evaporate enough water to move the equilibrium of the atmosphere toward a greenhouse effect: water vapor is an effective greenhouse gas, if you evaporate enough of it, it will keep the atmosphere hot enough to prevent condensation. You would still need several impact distributed in all the seas to be effective.
Your asteroid does not strike the planet. Your asteroid moves the planet.
Escape velocity for any gas depends on the size of the gas molecule and the size and temperature of the planet.
Check the pink line for water. Earth is above it so we have water. Venus is below it and does not. Venus is the same size as Earth more or less but is hotter than Earth which is why it is below that line.
Your asteroid is a big one. It does not hit which is lucky for everything down there because if it hit everything would be lava for a long time. It was a near miss. But in coming close it perturbs the orbit of your planet, moving it closer to its star.
Your planet warms up because it is closer to its star. As it warms up it moves to the dry side of that pink water line.
If you need it to hit for your story you can have some little baby buddy asteroids hit. The big one keeps going.