The quran was not meant to be a science textbook. Also, in this way, the verse can be interpreted in many different ways, which could all very well be true. It could mean, like I thought, that all living things contain water. It could mean that there is more water in a living organism than any other substance. It could mean that living things started out it water. All those are perfectly reasonable interpretations of the aya, and all of them are true (btw, clay is a bunch of clay material and water, so that interpretation does not contradict the fact that Allah created Adam from clay). So you've got several meanings all in 5 concise arabic words; quite awesome, in my opinion.

If the quran was going to be written like a science textbook, and include all those meanings it would be much longer, such as:
all living things contain water
there is more water in a living organism than any other substance
all living things started out it water.

And this is just talking about water, something that is mentioned one, or a few (not sure, exactly) times in the quran. Imagine every time Allah talked about the embryo or the heavens and earth he had to go through every single thing meant? The quran would go from being a book to being volumes and volumes; plus, it would turn into a textbook, which it wasn't meant to be in the first place. It is a science book, a history book, a theology book, a math book (if you consider the number miracles as math), a biology book, and I could just go on and on. With short, to the point ayas, Allah conveyed several meanings. Maybe that is why you find it ambiguous.