"I am always thirsty" can be translated as:
* Saya selalu haus ==> saya (I) selalu (always) haus (thirsty)
* saya selalu merasa haus ==> saya (I) selalu (always) merasa (feel) haus (thirsty, intransitive verb)
* saya selalu merasa kehausan ==> saya (I) selalu (always) merasa (feel) kehausan (the state of being thirsty, noun)
You can use the same sentences as above and make it colloquial by changing the subject you use and add some exclamations, such as:
* aku selalu haus ==> aku(I) selalu (always) haus (thirsty)
* gue haus terus nih ==> gue (I, slang) haus (thisrty) terus (keep) nih (an exclamation, slang)
You use "nih" only in a very informal conversation and can be used in many different ways. However, in the above sample, you use it to emphasize the fact that you're always thirsty, and also to attract other people's attention.
I've tried to think how to express that sentence in other ways, but we don't have many variations for it. So changing the subject and adding some exclamations are all you need to do to use it in colloquial speech.