(Apologies for the semi-threadomancy, but this required (and deserved) some thought...)
Ice and steam (and things like lava) all conspire to create headaches, no doubt about that.
RAW, ice and steam are listed within the basic Rego Aquam guidelines (123) - "Base 4: Change a liquid into the corresponding solid or gas (For example, change water into ice or steam.) This does not require requisites." However, the guidelines simply do not comment on whether "heat" is created or destroyed by that change.
However, nor is Ignem included in any Creo Animal or Creo Corpus effect that creates those living beings, yet they must be "warm" if they are “alive”, correct? If an Animal mage were caught in a blizzard, and wanted to create live animals to keep warm - would that not work? No Ignem requisite, yet some additional warmth is clearly provided and created along with the magically created, living creature (imo, at least)...
From these examples and the above discussions, it’s fairly obvious that the relationship between ice, water and steam simply refuses to fit neatly into the Hermetic basic system, and so we are left with either work-arounds to explain it, or some semantic sleight of hand to reconcile the model.
From reading the core book, intuitively*, anything that creates significant heat or cold (the destruction of heat in the Hermetic model) would require Creo Ignem or Perdo Ignem - that's at the core of the given definitions of Techniques and Forms. Creo creates (or heals), Perdo destroys, Ignem is warmth/heat/cold.
(* and I’ll admit this is problematic, because “I”, along with most SG’s and Players, don’t have more than perhaps a “0” score in Hermetic Theory)
So where does that leave us?
Exactly - it is clearly not Muto because a water-to-steam/ice effect is not an unnatural transformation - Rego covers natural transformations (in 5th ed, at least). (It is a magical transformation, but not unnatural - otherwise any Rego change that is not completely natural in time and progress would require Muto.)
Natural ice is necessarily cold, not magical ice. Yet magical ice that is not cold is not “natural” – and we’re back to Muto…yet ice and steam are natural states of water… ugh.
By the RAW, this counter-argument has just as much validity as the other, but let's look at some other clearly valid Rego effects that change things in an "unnatural" fashion:
o Turning a ferocious lion into a submissive 200 lb pussycat, eager to be scratched behind the ears, in a heartbeat is not "natural" - yet Rego magic can achieve this. Ferocious beasts do not naturally obey the whims of humans, yet this is not a problem and does not require MuAn.
O Changing a calm pond into a rushing wall of water is not natural, yet...
O To Levitate a target with Rego is not natural. True, to move them to X location may be natural, but to hold them there, suspended in air, is not – yet we are not Muto’ing the target to have no weight (which would be a different route to a similar effect).
O To teleport a target thru a solid wall or narrow bars is not natural, not without Muto'ing the target into gas or a narrower version of themselves, yet...
O turning a tree into a self-willed, fully conscious juggernaut is not natural...
O It is not "natural" for a person who just slept 12 hours and is wide awake to fall instantly into a deep slumber, yet this does not require MuMe...
Now, admittedly, none of these examples are exactly the same as "room-temperature ice or steam" - but they are not completely dis-similar, either. Both contain elements that are appropriate to Rego as defined, and elements that are not strictly found within the elemental nature of the target nor strictly covered by the Tech/Form combo “Rego-Form”, or not if we exclude everything arguably unnatural.
The alternate problem that the "steam = heat" solution creates is that if heat is created or destroyed, it breaks a different (and, to me, more elemental*) Hermetic principle, one more core to the system. If Rego Aquam creates hot steam, then Rego Terram would create searingly hot liquid metal, or insanely hot lava from stone with no Ignem req, right? Why bother with MuTe to turn stone to mud to trap something when you can Rego the same stone into lava and simply carbonize the victim out of existence. Using Rego to separate alcohol from beer, it should heat up both, because distillation naturally takes heat, otherwise it would be Muto – heatless distillation. And suddenly we have steam heating without Creo Ignem, iceboxes without Perdo Ignem, lava flows without any Ignem at all... and that's not how I understand Hermetic Magic to work.
(* no pun intended)
In short, it comes down to whether you believe, within your Saga’s/Troupe’s understanding of Hermetic theory, whether it is easier to ignore the “unnatural magical ice/steam” paradox than it is to ignore the “Rego alone can’t change water to a different natural form” paradox, and then live with the subsequent limitations that either ruling creates, the echoes of that ruling in parallel situations with other Techs/Forms.
(You could also require requisites, but that becomes impractical - then ice and steam become substances of great difficulty for Hermetic magi to conjure from water, and that just doesn't feel right either.)
For me, in my sagas, since hot steam or cold ice would require CrIg or PeIg, magical ice or magical steam are the same temperature as the water they started from or the day they’re created in - it's magical. The ice might be a bit cooler but not cold, the steam is muggy but not suddenly hotter. Magical ice does not melt faster on a hot day or slower on a cold day - the Rego effect is what holds it as ice, not the temperature. Fire or heat do not affect Rego ice any faster than it could evaporate the natural water (which is "almost not at all").
(I also require +1 magnitude for Ice (not steam) in the same way that some other Forms require it for more valuable/dangerous/desirable examples of that Form – ice is just more powerful than water, and some balance needs to be maintained imo. The diff between creating a cubic pace of water in the air, around a target or elsewhere vs. creating a cubic pace of ice speaks for itself.)
But, again, this is just one SG's solution to an unanswered question - ysmv.