Underwatering cause more males

No. Male or Female (Dioecious) is already determined the moment the Egg is fertilized. or Monoecious both Male, and female organs on the same plant.
Drought could cause stress, and make a female become Monoecious.
 
Water will not determine sex. Now, heat and light stress can cause a plant to herm. Make sure your temps and humidity levels are good. 72-76 with a 10 degree drop at night. And keep humidity at 60-70% during veg, and like 30-40% in flower.
 
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