A tile of water with a depth of 1 will freeze to create an ice floor. A water tile with a depth of 2 or more will freeze into an ice wall and will additionally create an ice floor one Z-level above it. The ice wall will be the same no matter how deep the water is. As there is no thin ice, the floor can be walked on freely. Ice walls can be mined like any other natural wall. The ice wall will melt into water of depth 7  when warmer temperatures arrive, which can keep ponds full if rain only fills them up to 2/7. An ice floor will also melt, leaving 1 water if it was smoothed or engraved.
Entire water supplies can become completely frozen upon winter in colder areas. This can depopulate an entire fortress with rapidity in the first winter if they're unprepared. To counter this, make a cistern inside beforehand or brew enough alcohol.
 Ice as a stone
Ice appears as a light blue stone which can be found by mining through an ice wall. Ice can be used to build constructions and workshops. Ice boulders will melt when exposed to warmer temperatures (such as inside a fortress), giving it rather limited use. Nevertheless, workshops made of ice have a certain novelty to them, and it's even possible to make furnaces out of ice, as counter-intuitive as that sounds.
The game refers to ice boulders as "water." It does not appear in any stockpile options or the manager, so it cannot be moved by designating a stockpile, although it does appear under the "stones" section of the Stocks menu. Ice can be moved by dumping it.
It is possible to sell ice to merchants.
 Icy constructions
All constructions are entirely indestructible. Entire fortresses can be built of ice in temperate climates equally impervious to catapults, the summer sun, or a thousand tons of boiling lava. If constructions of ice are dismantled during the warm season, they will melt into thin air.
 Ice as hazard
Watch out when digging through ice into (subterranean) unfrozen water resorts. The space cleared by the miner will freeze solid again instantly, encasing the advancing miner into a wall of ice. This means not only the loss of a valuable dwarf, but also of his now inaccessible equipment.
Additionally, surface ponds which go through a freeze/thaw cycle (on Temperate and Cold maps) can be hazardous, as your dwarves will treat the ice as a normal surface when moving about. Spring thaws happen suddenly and without warning, and dwarves in the midst of crossing a frozen pond will find themselves suddenly swimming in depth 7 water. You can avoid this by marking restricted traffic zones over ponds in high traffic areas.
 Melting outdoor ice
It can be very important to be able to extract water from permanently-frozen ice (for instance on cold northern maps) in order to give water to the wounded. This can be done with magma. Dig out a tunnel one z-level below the ice sheet and fill it with magma. The ice on the above z-level will melt. You can see this happening here: http://mkv25.net/dfma/movie-153-meltingwateronglacier. Stationary magma below surface water does not affect the freezing of the water. However any change in the amount of magma in the tile below the ice will melt the ice (which will soon freeze again), so flowing magma will keep surface water mostly usable in biomes with a cold winter.
If you would like to transport magma to heat a frozen brook or other such feature that is very far away from the magma pipe, try to use a magma duct that is nearly as wide as the area you intend to fill. Otherwise, magma will tend to evaporate as it disperses from a narrow duct to a wider area.
 "Melting" ice without magma
It is also possible to get water from ice without actually melting the ice, though this method is hazardous and destructive. First, dig down to a non-ice layer beneath ice layers and dig out a room. In the layers directly above the room, dig rooms out of the ice with the same dimensions as the first room. Dig channels in the ice floors around the entire perimeter of the room EXCEPT for one square right next to the hallway (this is important; if you don't leave the last square accessible from outside the room, you will likely kill or injure the miner doing the channeling during the last step).
When you channel out the last bit of ice, the entire ice floor will cave in down to the the area you already cleared out. Depending on the stupidity level of the channeling miner, he may simply be stunned or also plummet to his doom. The broken ice will melt at the bottom level, but the floor will also be destroyed, leaving nothing but "Open Space" with water floating on top of it. The newly-melted water must then be redirected to another location (one square of broken ice floor yields roughly one level of water wherever it lands) - to avoid drying out, multiple ice floors must be dropped in order to achieve sufficient water depth.
Don't expect to create farmland in the original room. Since the dropped floors destroy ground tiles on impact, water must be redirected to an area with intact floor tiles. Floor tiles must be constructed within the caved-in region in order for the area to be used for other activities.
Alternatively, by clearing out an additional Z-level, you can drop entire ice walls into the bottom room, instantly filling it to 7/7 depth. However, in this case, the underground chamber should be several Z-levels below the ice, or the cave-in will cause water to splash up into the ice area, freeze, then cave-in into the water, beginning a potentially endless chain of cave-ins.
Also note that if there is empty space underneath the destination floor (other rooms, hallways, etc.), the falling ice will crash through that floor as well. Obviously, this can be dangerous. While a dining room full of hungry dwarves will certainly appreciate new farmland despite the frozen wastes above ground, those hungry dwarves will also end up angry, wet, and dead when several tons of ice come crashing through the ceiling. On the upside, fewer mouths to feed.