At age 9, I drew an alien which I described as "eating ice". Occasionally I've wondered if that's possible. Carbon-based Earth life only uses water as a solvent and consumes carbon compounds for energy. Is an alien biochemistry possible which consumes water for energy?
EDIT: And not carbon compounds (so no, photosynthesis doesn't count).
Plants consume water for their energy metabolism.
Water is more than a solvent for a plant. It is one of the 2 raw materials they use to make sugar. The formula for photosynthesis is CO2+H2O+ light -> CHO (sugar) + O2. Plants do chemistry on water molecules, splitting the H2 from the O to trap the incoming energy from light. Without water chemistry there is no photosynthesis.
Plants (and us) then reverse that and combine CHO with O2 and release the energy. If you wanted more straightforward water-only energy metabolism, you could use sunlight (or some other energy source) to electrolyze the water to H2 and O2 then have your animal recombine those back to H2O in such a way as to recapture the released energy as ATP
If photosynthesis is considered cheating (!) and you just want to use the energy in pure water, you could have your creature consume hot water. Water is sometimes available at temperatures considerably above ambient - geothermally heated or heated by the sun. You could have a creature which drank hot water and used the energy difference to effect a conformational change in proteins. When the proteins relaxed back to their normal low energy state, they could turn a molecular wheel and generate ATP.
It is extremely unlikely that there would be such a biochemistry. Water is very stable molecule because it's the "ash" of burning hydrogen, so there isn't much energy left in the molecule that can be liberated.
Now, you can oxidize water as fuel, but to do so you need an extreme superoxidizer, such as chlorine trifluoride*. But when you get into chemicals like these, they are basically so reactive that there is no stable biochemistry that could form around them.
*: Famously described to spontaneously ignite on contact with any known fuel without measurable ignition delay, and also able to burn other extremely incombustible things like sand and asbestos, not to mention test engineers.
Is an alien biochemistry possible which consumes water for energy?
In theory, you could have a biochemistry that used fluorine in the way that we use oxygen. Fluorine reacts with water to produce hydrofluoric acid, oxygen and some energy, so such a metabolism could "eat" water. Thing is though, you'd get a lot more energy eating pretty much anything else... have a look at videos of fluorine gas reacting with various other materials (spoiler alert, it involves a lot of things catching fire). You'd also need to find some mechanism for producing fluorine... the damn stuff is so much more reactive than oxygen that even if you did have a fluorine-producing photosynthesiser you'd have a hard job finding an environment where it would persist in an atmosphere for long, and if it did it would rapidly react with any nearby water.
Still, the possibility of fluorine-based life has been considered, and doesn't appear to be totally implausible. It probably wouldn't "eat" water, though.
There are two concerns here: Energy and Materials.
Energy Is covered pretty well by @Willk. I just want to reaffirm that the bonds in water could be used to both break and make the bonds of ATP depending on the enzymes at play.
Materials However, I think the bigger issue is materials. The average human is made of Carbon (20 kg), Ammonia (4 L), Lime (1.5 kg), Phosphorous (800 g), Salt (250 g), Saltpeter (100 g), Sulfur (80 g), Fluorine (7.5 g), Iron (5 g), Silicon (3 g) and fifteen traces of other elements. (I'll find the citation later). The point is that a person is made of those things because they eat those things. What are your aliens made of? Even plants absorb sulfur and phosphorous from the ground and carbon from the air to build themselves.
If you want your species to only eat water, could I suggest making it silicon-based and drinking lava for materials? This would provide all the base materials for an organism.
Such a world, with solid ice and liquid rock side-by-side would be an interesting place.
Possible, and the other answers miss how ice can "contain" energy.
As water freezes, dissolved salts predominantly remain in the liquid phase. If you melt this ice, the resulting water will be relatively pure. Now it turns out that for entropy reasons, you can extract energy from mixing pure water and salty water.
Hence, we may have an animal that lives in a cold, salty lake. It drinks salt water and eats ice, extracting the energy released from mixing it. The reason this can work is because the ice is formed at night when it's colder, and the animal sits in the sun during the day to melt that ice. Hence, the animal effectively lives off the energy from the temperature difference between day and night.
The key here is that it's not just the ice that contains the energy, drinking salt water is also necessary.
The alien would have to have a very different chemistry to ours. And live in a very different environment. Quite radically different.
There are some chemicals that can react with water to release energy. It is just barely possible that such a chemical basis could support an organism. Under the right circumstances, for example, various metals will take the oxygen out of an H2O molecule and release heat.
It would have to be a very different chemistry to ours. And they would need to obtain several other chemicals besides water to sustain themselves.
So to build a little more of an ecology. Imagine a plant that absorbs sunlight and converts something like aluminum oxide and some hydride compound into pure aluminum and water. It might then release the water, possibly as ice crystals if it is cold, and use the aluminum to build its structure. The ice-eater could then come along and use the plant as one part of its food, and the ice as another part.
Conceivably such an organism would be radically susceptible to various chemicals we take for granted. It might spontaneously combust in the presence of free oxygen, for example. (Hmmm... SE's dictionary does not know the word combust. Hmmm...)
Now there is just a huge amount of chemistry that would have to be worked out to make that work. It would involve a lot of things like acids and catalysts and ways to store such things. So, whether it is in fact possible, or even vaguely likely, I certainly can't say.
On the other hand, carbon chemistry also has a huge amount of complicated and subtle reactions that are required for the kind of life we know. If I was required to sit down and work out if carbon-based organic chemistry was possible, given I was only familiar with the conditions in Saturn's rings for example, that would be a daunting task.
Unlikely.
Water does react chemically with many compounds. Alkaline metals, some halogens and certain compounds are the exceptions. Of those, only fluorine can make a plausible atmosphere that a creature can breathe to oxidize water. But free water would not be able to coexist with fluorine atmosphere over long periods of time. As @puppetsock pointed out, this whole ecosystem would be very problematic.
However, water can produce energy without having a proper chemical reaction. Just hydrating and dissolving many compounds can yield a noticeable amount of heat. So I can imagine that the world this creature lives in is a desert. The creature would eat some dry salts and then drink water. Salts would dissolve in the stomach, producing heat, which can be used as a source of energy. However, this restricts energy generation to the stomach.
Cell-level metabolism would be difficult to imagine. In humans and animals, nutrients are transported to individual cells by blood, and energy-producing process occurs there, inside the cell. If we want similar process here, then this hypothetical creature would need to have a transport mechanism which can move dry salts from the stomach.
This creature could "eat water", although water would not be what provided its energy. It could break down the water into hydrogen (used as a fuel), and oxygen (used as an oxidizer). It could have a digestive system more like the fuel consuming machines we have. it separates the hydrogen from the oxygen via electrolysis, with the electricity coming from solar panel-like structures.
