From reviewing the way water is treated in the game, I've come to a few conclusions. For one thing, it seems that "it's an alpha!" is a big part of this issue. Handling water as an item hasn't been polished at all, and we can expect great changes in the next release.
Conclusions:
1) As most readers will know, there is a material, be it stone, metal, or water, which is labeled without a capital letter and is usable as a building material, among other things. Examples are "water", "copper", and "microcline".
2) There is a second form for all of these (except bauxite, which never melts) which is the melted (or sometimes melted and cooled) version, which has a capital letter, "Water", "Copper", and "Microcline". This is usually waste, except in the case of "Water", which can be collected and stockpiled as a resource for your dwarves. All are moved with the quirky "Cleaning" labor.
3) "Water" can freeze, forming "ice", which does not follow these normal rules, but is in the same classification as "grass", simply an ambient material with which the game's temperature and environment may react (only, ice melts and grass burns).
4) "Water" is never actually IN the bucket as a container; in fact it merely occupies the same square and is hidden until delivered or consumed. This is seen with a stack of "Water ( 8 )" beside a wounded dwarf's bed when he is brought water and the bucket is moved.
5) Drawing "Water" with buckets from a non-freezing z-level into a freezing z-level nearly instantly creates "ice" on the square the "Water" occupies. Due to the way "Water" is handled, traveling through any freezing square will cause it to become "ice" on the floor.
This problem was not supposed to come up, as dwarves will not attempt to use a water source in a freezing area unless it is designated as a source of water. I wanted my #@!@# engraver to get some water, so when I noticed they were not taking from the hot spring I used "i" to designated it as a source of water.
In the end, none of this should be an issue when Toady gets around to smoothing out the water/ice/melted materials/cleaning/heat transfer/melting point problems. All of which are known quirks. For now, maybe a line or two in the Wiki alerting players to these quirks will suffice.
Edit: Placing 8 in parenthesis creates an annoying unwanted smiley. Added spacing.