I'm searching for a book I once read where the main character (fairly sure it's a man but could be a boy) is stuck in an inhospitable environment, a desert I think. He is days away from civilization which is also the nearest water source. Luckily, he finds a creature that appears to be made of water, it's translucent and starts off around the same size as the main character. The creature offers the man assistance, allowing him to drink from it and staying with him on his journey (guiding him to the town?). As he drinks the creature gets smaller and smaller.
Eventually he can see the town in sight but the creature has only a few mouthfuls of water left in it's body which it gives to the man so that he will survive. I think the man promises to come back for the creature with water when he gets to the town, and drinks, yet when he comes back the creature is either dead or missing. I remember it being very sad!