So after mucking about with a few adventurers on the same area of the ocean, There are indeed what appear to be either giant upheavals of ice, or freezing "waves" of ice that move across the surface. Large portions will appear and disappear in a single move, and claimed several of my adventurers. The large cracks appear to go all the way to the ocean floor, but there are some irregularities in them as well. They tend to be a uniform 1-tile wide crack for a few levels, then you can see a split, like a cross partway down. The crack then returns to being a uniform, 1-tile wide space until you hit the bottom. Occasionally, there are visible offshoots from the surface level opening, but I've never found one that can be walked-through. I sent a few adventurers off of the cliffs to see what was below. None survived.
Waiting or sleeping has had no effect other than my adventurer being encased in ice. However, it is always encasement in ice, and never freezing to death. This leads me to believe that these "waves" occur fairly frequently.
Chances are they are glaciers, but its wierd because they aren't listed as glaciers on the map. Initially I thought they might be icebergs, but that would also be silly because they are, again, ice all the way to the bottom. I suppose if they thaw then they could be listed as oceans.
Either way it would be cool to study how they work a bit more.