Being thirsty in a dream could be a terrifying and worrying experience that could have different meanings as per Islam.
1- According to Ibn Sirin رحمة الله عليه the greatest dream interpreter of Islam, thirst is not a good sign. It is better to be unthirsty.
2- If someone is thirsty and is unable to find water, the meaning of such dreams is sorrows related to either world or religion.
3- If a dreamer was thirsty in the dream and someone gave him water that he refused to take means that his sorrows and worries would end soon and that he shall attain his wish.
4- According to Jafar Sadiq رحمة الله عليه if a thirsty man drinks water in a dream it means that he will commit a sin, yet in the end, he will repent over it. Ultimately, he shall become a pious man.