@atticusblame you asked about water...
On a higher carb diet the energy is stored in our muscles and liver as glycogen (and then when the glycogen stores are full, the energy is laid down as fat). Then when we exercise, or get hungry, the glycogen stores release glucose into the blood stream.
Glycogen storage is also linked with water retention - biochemically every gram of glycogen is associated with 3g water.
At the start of a low carb diet the glycogen stores are being depleted and not refilled, so you loose the associated water (hence some of the peeing we experience). You therefore lose some weight.
On a calorie restricted diet the same would happen BUT in that case as soon as you ate, glycogen would be created and the associated water would be retained. (that's why people can crash diet, lose a few pounds to get into a special dress, then put it all back on the next morning).
With a low carb lifestyle once the glycogen has been used up our bodies turn to fat for energy and start using that up (hurrah!) That is what we are aiming for - the ongoing use of fat (eaten fat and body fat) as our principal source of energy.
So losing a lot of weight fast in the initial stages might be due to water loss but it's an important step on the way to longer term fat loss. And you'll only regain that lost 'water weight' if you abandon this way of eating and return to carbs.
You may have lost water from your body but rest assured that unless you are dehydrated, the water you are losing is from its biochemical association with glycogen, and drinking more water will not replace that (unless you're eating carb and refuelling your glycogen stores).
In addition, the biochemical process in the liver that converts fat requires lots of water to make it happen efficiently, so keep on keeping on with the water consumption.