both work when you feel need a drink.

ke is a word, kou ke is a phrase.

Ke is the short term of Kou Ke. I list a corresponding example here,chemo-chemotherapy,they are the same thing. So the full translation is "kou ke",but "Kou" stands for mouth in Chinese,when you say thirsty,you don't need to mention that it is your mouth feel thirsty.Therefore,we say "ke" the same.

渴 口渴 都是一个意思 you can say : wo ke le or wo kou ke le (theey all means (i am thirsty )

that's all right~