The following is the sign for "drink" but we have been using it for "thirsty". I don't know that it really matters. Any thoughts on that? Some signs have more than one meaning, such as the sign for "all done" being synonymous with "all gone". To babies it means the same thing.
In any case, I think it gets the point across. I'm happy that Blueberry loves water. She doesn't drink a lot of it but she likes to drink from a cup, especially Mommy's cup. She makes it clear when she wants a drink, but helping her learn a sign can help others besides her intuitive Mommy understand what she wants.
To sign "drink"/"thirsty" you form a "C" with your hand and point your thumb towards your mouth like the spout of a sippy cup or a straw. I have also seen it performed as just a "C" brought towards your mouth like you are bringing a cup to your mouth.
Babies aren't always precise with their signs so while they are younger you have to watch closely for them while they are still learning. As they get older they become more dextrous and are able to perfect their sign, just as when they start to speak they get better at enunciating as they get older.