Laticauda colubrina

Photo by Jens Petersen

The Colubrine sea krait has got to be one of interesting aquatic species of animal, from my perspective.  The Laticauda genus is one of the few, if only, among the sea snake family, to have any species retaining adaptations for both aquatic and terrestrial activity, and as evidenced by the photo, right, is putting on a top-notch marketing campaign.
Various species within the family are known to or thought to possess social behaviors (including swimming alongside certain fish species), the ability to remain submerged for hours (they must surface to respire), are often highly venomous (much more so than terrestrial snakes), yet almost all species docile towards humans.  Many of these species have convergently evolved the ability to breathe in a limited fashion through their skin, which is a trait most famously found in amphibians.
L. colubrina is certainly an excellent example of how morphological and physiological traits otherwise found fish, amphibians, and various mammals can, by means of independent evolution, result in the creature we see today.