The accepted answer above is pretty good, but I'd like to debate just the dot point on ice and snow (which might be better if I could instead answer the specific question Does the Shape Water cantrip work on snow or ice?, but that is closed and redirects back here).
You choose an area of water that you can see within range and that fits within a 5-foot cube.
Water comes in multiple forms: vapour, liquid, or solid. Ice and snow (and steam and mist, etc) are water, so they can be targetted. If they can be targetted, then any of the dot points of the cantrip are applicable even if they work against our preconceptions of what's plausible (because magic). Case closed.
Well not so fast. While the above might be sufficient if you happen to agree with me already, from scouring the internet I can see that this is surprisingly controversial and it all comes down to language.
When it comes to rules Jeremy Crawford has said: "Unless the rules explicitly expand, narrow, or completely redefine a word, that word retains the meaning it has in idiomatic English". I confess that the term idiomatic isn't actually idiomatic itself for me, so I'm going to go with 'natural' instead, as with this particular definition (first one I found) from the Cambridge dictionary: Idiomatic also means natural in expression, correct without being too formal.
I think differing understandings of 'natural language' form the crux of most disagreements people might have over the usage of Shape Water. It may be that the most natural interpretation when using the term water without context would be liquid water, a usage with a narrow scope. But it would not then follow that this particular interpretation must then apply, especially if there are other natural ways that the term can be used, such as 'water' being used more categorically, a usage with a broad scope. Less natural does not equal unnatural.
But I'll go a step further and argue that we naturally use the term water categorically, most often. We have specific terms for water in its frozen and evaporated states: ice and steam, so a reasonable arguement might be that if the designers meant other forms of water to be targettable then the designers would have written those other forms into the description (ignoring that the corresponding counterpoint works just as well, that the designers could have added in 'liquid' to the description). Water is incredibly common so we have many different terms for it beyond just state change, and listing what is an acceptable target would take days: dew, ocean, puddle, geyser, rain, lake, marsh, reservoir, river, boiling, ice-cold. Water that is big, small, moving, collected naturally, collected for a purpose, at varying temperatures, and on and on. All specific terms that we use naturally over the generic'water', but none that I can imagine any would argue don't naturally fall under the category of water, or that would raise any concern if targetted with by the cantrip Shape Water. So why has the state of water been focussed on, is there anything in the spell that singles out form?
If we instead looked at unnatural usages, we could start by considering another spell: Shape Stone. We wouldn't think that it targets ice (with the reasoning being in some circumstances ice can be classed as a mineral and minerals are stones)(my science on this is much less certain, but my arguement doesn't hinge on getting the science right). Likewise we don't think that the spell Water Breathing would work in all situations simply if we held a waterskin over our head, but only when underwater by submersion. Or if you wanted a glass of water and received a glass of ice you would reasonably consider your request unfulfilled. Not because you didn't literally receive a glass of water, but because that request has a clear and exclusive intention in idiomatic English: water for the purpose of drinking.
These examples either require a narrow, technical interpretation that uses specialised knowledge (or going to wikipedia); or they twist a term into something it doesn't mean; or they have one clear meaning (even if RAW they could be interpreted otherwise separate to context). That's not the case with the shape water spell.
So there are multiple factors that go into how we use language naturally. But I can only conclude that if ice and snow are water, and if the term water can naturally be used categorically to include state changes, and if the distinction for separating out ice and snow is only arbitrary, then Shape Water can be used on ice and snow. If I paraphrased JC's quote from above into my own terms to support this, it would be as so: "Unless the rules explicitly narrow the usage of the term water, water retains the [full breadth of] meaning it has in [natural] English".
Thanks for reading this far. Outside the language discussion above, here are a few other factors that support or otherwise don't contradict allowing Shape Water to be usable on ice or snow (etc).
- All the above can be discarded if in 5e there are discrete game terms such as Water(TM), Ice(TM), Snow(TM) that have specific meanings and can't be used interchangeably, or grouped together under the category of water. To my knowledge no such restriction applies.
- Sometimes rules trump logic, or science. But in their absence there is no reason to believe that real world physics are suspended unless explicitly confirmed to exist.
- And if we look at other spells clarity is given (or attempted) by the provision of lists of terms or modifiers added to the term water. Walk on Water includes the term 'liquid' (and can be used on snow). Control water has 'freestanding' water, 'standing' water, 'flowing' water, and 'body of' water. These might not actually make things easier to interpret but it's more than the generic term water that we have to go off here. Destroy Water clarifies that it can be used on fog (there's no reason to think it couldn't otherwise, but the dot point also increases the area of effect).
- RAI, I actually expect that the designers might have meant liquid water. However RAW is not RAI, so without errata RAI is superflous for RAW based games.
- I've seen mud and blood brought up often as counter-arguements, but I view this mostly as a distraction. Mud and blood aren't water; while they contain water they're a separate substance. This is not the case for ice etc (not all water is ice, but all ice is water).
- And not a great arguement but we actually have ice being referred to as water in this spell with the wording "the water unfreezes" in the last dot point. (And on this dot point ice remains a valid target, as freezing gives an extra effect: it can't then be unfrozen for the next hour).
- And introducing restrictions based off state might have further complications, if taken to absurd extremes. Do spells that target creatures have variable effects or simply not work depending on the state of the creature (with elementals coming in Air (vapour), Water (liquid), Earth (solid), and Fire (none of the above) forms)? Or what if a monster changes state, do different rules suddenly apply (like a vampire assuming a Misty Form)?
