"Do you think that she is thirsty?"
Translation:Myślisz, że ona jest spragniona?
Sentence "Myślisz, że chce jej się pić?" should be also correct. Native Poles say it both ways.
That is technically quite different, but functionally it's the best translation. The word "spragniony" is used a looooot less often than English "thirsty". "Chce mi się pić" is the most common way of saying "I'm thirsty".