I have noticed this phenomenon in myself. I find it difficult to lose weight when I am not hydrated.
Check out these articles:
Dennis EA, Dengo AL, Comber DL, et al. Water Consumption Increases Weight Loss During a Hypocaloric Diet Intervention in Middle-aged and Older Adults. Obesity (Silver Spring, Md). 2010;18(2):300-307. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2859815/
Thornton SN. Increased Hydration Can Be Associated with Weight Loss. Frontiers in Nutrition. 2016;3:18. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4901052/