These water based octoroks can be annoying at times, especially when encountered in the fog. They stay close to the water and dive under repeatedly, shooting rocks when they reemerge. A single well aimed cannon strike will defeat them.
Ocean Octoroks will appear underwater and will shoot ink at the Spirit Train making it much slower. The ink will also make the map disappear for a short period of time. Rub the screen to remove both of the effects. Otherwise, they aren't much trouble, besides the amount of hits they take to kill. They commonly drop 20 rupees when defeated.