To treat dehydration, what is better, water or a sports drink?

Depends: If a person is exercising for a short period of time and the exercise is not intense, regular water is fine. If a person exercises for longer periods of time (marathon, ironman, etc), then it is important to drink fluids containing electolyes and sugar (to aid with absorption of sodium). If you're dehydrated from an illness and able to drink without vomiting then use an oral rehydration solution.

It depends: For mild dehydration, water is fine. For severe dehydration, often due to severe diarrhea/heat exposure it is important to include salt, sugar, water, and other electrolytes. Sports drinks often have these ingredients but typically have more sugar than is recommended, particularly in the setting of severe diarrhea. Salty chicken or vegetable soup is often a good initial solution.

Oral rehydration: The best fluid for dehydration is a fluid that contains the correct balance of salt and sugar because the bowel absorbs the water along with salt and sugar in a specific ratio. In the usa, pedialyte is the most common brand, but there are other brands too. The world health organization has its own oral rehydration solution that is used predominantly in the developing nation.

Sports drinks. : During vigorous sport participation, one loses fluids and electrolytes so you'd want to replace both. As such, sports drinks would be better during sport participation.