Antarctic octocoral, Paragorgia sp. photographed in Stromness Bay, South Georgia, where Sir Earnest Shackleton completed his heroic journey to find rescue for his men. This delicate species is fairly uncommon at the sites I dive.

A large crinoid, Promachocrinus kerguelensis; fairly common around the Antarctic Peninsula. This individual is about 80cm (32in) across, swimming with a beautiful coordinated motion of its arms.
Shortlisted!

Clione limacina, a pteropod or sea butterfly, photographed in Hornsund, Spitsbergen. There was a bizzard of zooplankton in the water that day - you can just make out some calanoid copepods in the water around the pteropod.