Orca (Orcinus orca) cruising on the surface somewhere in Southeast Alaska, USA.
Orcas are so sleek and hydrodynamic, and so consummately evolved for speed and distance. I always marvelled at how they slide through the water with such effortless ease, barely creating a ripple: even the surface tension of the water appears to stretch over their head like a second skin. An orca's dermis (skin) is smooth: the outer layer continually and rapidly renews itself, and the old skin sloughs off. The increased skin cell turnover increases swimming efficiency by creating a smooth body surface which reduces drag.