They are both wonderful.
A Drop Of Water by Walter Wick says right on the cover it's a "book of science and wonder." Page after page of stunning photographs fill this book, each showcasing a fantastic property of water and complimented by text that challenges a young reader (or listener) to learn more about the scientific properties of water than the typical children's book (surface tension, capillary attraction, surface area, molecules, evaporation, condensation, cloud formation, refraction, and wavelengths, among other topics). First Son, thanks in no small part to the reading we've done earlier this year, understood quite a bit of it. Even if a teacher or parent just summarizes the text a little while children gaze at the pictures, it would be a valuable book. At the end of the book, the author suggests experiments that could be conducted at home, many of which we did in the course of our study (or similar ones). The pages on snowflakes would be well combined with a reading of Snowflake Bentley by Jacqueline Briggs Martin (a book always worth reading).
A Drop of Water by Gordon Morrison is a different kind of book, but just as delightful. The Author's Note describes the book very well:
This is a story about a moment, the time it takes for a drop of water to fall from a child's fingertip. Water is the thread that takes you through the story; from the child, to the clouds, to the mountaintop, then down through forest, woodlands, streams, ponds, and back to the child by the meadow brook. By following the thread we travel through a wondrous landscape and pass plants and animals, including the child, doing what they do: soaring, drinking, eating, blooming, nesting, playing--living, all within the same moment.The illustrations are wonderful - life-like but only partly colored to emphasize the plants, animals and water of each habitat. The text is not overwhelming, describing each spread engagingly but keeping the focus on the illustrations. At the end of the book, short descriptions of many of the plants and animals appear (perhaps all; I didn't count them). My children adored this book. None of them could keep away as I was reading it aloud.