Ah I see. Yeah, that's a pretty valid reason not to have it, if it would indeed be that difficult. Though I don't know how difficult it would actually be. Doesn't seem like it would be, all you would be doing was adding a "frozen water" mesh to the area the player casted as soon as it hit the water. Seems like something people would even be able to mod in. And probably even have an animation of it sinking and disappearing.
I've gotta disagree. I think to do this as several have suggested would be incredibly difficult.
Sure if we just want cold spells to have some generic extra pixels of frosty whiteness appear when they hit water, yeah probably not too hard. But that would be pointless.
The idea of actually having the spell create some mass of ice? Much harder. 1st it wouldn't sink, ice floats. They would have to create a new object with appropriate visuals, then give it proper physics (such as floating on water, but sliding along stone pavements if we got some in a street). And that (which I see as utterly past any physics in Oblivion), gets us ice in a minimally interactive way.
Add in the effects some mention, such as freezing enemies in/under ice, or ice melting over time, or ice melting from fire...and we're talking about a very large amount of coding. All ice would have to be assigned a sort of "cold points" that decrease over time relative to the temperature in the area it's located (we would of course want ice to melt quickly in a blacksmith's shop but very slowly if at all on a snowing mountain right?). Then how size changes as it melts. Also the damage done to enemies who are encased would need to be determined and kept track of.
No, in the end I think the only practical implementations are fairly pointless, and the interesting parts are beyond Bethesda or any game makers at this point (not of course if they really wanted too, but the time required to make ice realistic would be much better spent elsewhere).