Icebergs can be found in the North Atlantic and are plentiful. You can destroy icebergs by shooting them with any of the Morrigan's weapons. You can run into them, but you will lose health that way.
During my first playthrough, I forgot to shoot them, and only had 20/100 at the end, and had to grind them out. My advice is that whenever you're near one, shoot it. Also, shooting one in combat creates waves that will stun and damage other ships.