You would think that sea ice would be pretty smooth. Again not so. We were given a one hour lecture about snowmobiling when I first arrived and that was the extent of my instruction. Sea ice has what are called pressure ridges where the two edges of different areas are pushing together and form rising ridges. The big ones you can see and avoid, in most cases.
Snow covers much of the ice where it has blown in.
The snow forms mounds and valleys which cannot be seen very easily. At a nice leisurely speed they are easy to ride over. I had to make the latter choice. Believe me some of the ridges and mounds had surprises for me. I would go up and the snowmobile would go down or vice versa.