Carbohydrates (especially whole grains) store water and if you eliminate carbs and grains, you may need more water, especially if you work in the heat.
According to paleoleap.com, one important consideration for low-carb dieters: salt.
Sodium, potassium and magnesium are electrolytes and they maintain fluid balance in the body. Since low-carb dieters eliminate salty processed food, they don't get heavy sodium doses and may need more.
In addition, low-carb diets are low insulin diets. When insulin levels are low, the body flushes out more sodium, according to physiology.org.
Don't be afraid to salt your food when on a low-carb diet. If nothing else, use light salt as this type of salt has more potassium.
Salt or electrolyte tablets can be an option if hydration is a problem.
Bring a thermos of broth or bouillon for lunch. That can help, too.