Insulin causes water retention. When you keep your carbs low, you also keep your insulin low, so you don’t retain as much fluid. That’s a good thing, especially for your blood pressure.
But it also means you may lose electrolytes more quickly than usual. Most people on keto take extra salt and magnesium. (I take pink salt when I’m fasting and Jericho plus magnesium every night. Without it, I get brutal leg cramps)