All snakes are good swimmers, but only the sea snakes have successfully evolved a suite of characters that have allowed them to move permanently from the land into the ocean. With the aid of a paddle-shaped tail and potent venom, they are efficient predators on a wide variety of fish and their eggs. They prefer shallow tropical waters, but a few species are found well within the temperate zone, and one species seems to be completely pelagic. The sea snakes are easily recognisable as they share many similarities with their terrestrial snake relatives.

• A paddle shaped, vertically orientated tail.
• Nostrils that can be closed underwater (valved).
• Small, specialised venom injecting teeth (fangs) at the front of the mouth, with powerful and fast acting venom.
• A salt gland on the floor of the mouth under the tongue to excrete excess salt.
• To give birth to live babies underwater.
• To be mainly restricted to warm, tropical seas.

Sea snakes look basically just like land snakes, but have a flattened tail paddle. Adults vary in size from half a metre to about two metres in total length depending on the species. Some have bizarre adaptations for feeding on specific prey: one species has a long, thin neck and tiny head on a large, thick body. This is an extreme adaptation for reaching the fish eggs that are its sole diet, and which are hidden deep within rock cracks and holes. Colour is very variable in the sea snakes, but a surprising number of species have regular bands or rings of colour around the body, presumably for camouflage, and perhaps to advertise that they are very poisonous or nasty tasting.

The true sea snakes live in tropical and subtropical oceans from the Persian Gulf across through South East Asia to the Western Pacific and Northern Australia. None are found in the Atlantic Ocean. One species lives in a freshwater lake on Luzon island in the Philippines, while all others are marine. Many species are demersal and live in estuaries, mangroves and on shallow reefs. Only one species, the yellow-bellied sea snake, is truly pelagic and can be found thousands of kilometres from land in both the Indian and Pacific Oceans.

Most species have toxic venom for quickly immobilising the fish, eels and crustaceans that make up their diet. In a few species that are specialised fish egg eaters, the venom and the fangs are degenerate and non functional. Other species have become highly specialised in their diet and only eat a single type of prey, such as eels, gobies or catfish. The most common species encountered in non-tropical areas, the yellow-bellied sea snake, travels along on the surface with floating debris in mid-ocean and eats the small fish that associate with these rafts.

Predators of sea snakes include sea eagles from above, and sharks, large predatory fish, eels, and perhaps even crocodiles from below. The surface dwelling yellow-bellied sea snake is believed to be unpalatable, as there are no reports of sharks, fish or predatory birds eating them.

The true sea snakes are the only truly marine reptiles, reproducing at sea. All other marine reptiles must return to land to breed. Fertilised eggs are retained inside the mothers' oviduct until they are fully developed, and all young are born live (viviparous). The new born young are independent from birth. Little is know about growth rates and survivorship in sea snakes, as little research has been done on this group.

Sea snakes are an important component of the fauna on many tropical reefs. They are usually forage on fish or eels, and some species are very abundant on shallow reefs.

Sea snakes are rarely seen by most humans, however fishermen in tropical areas are well aware of their toxic venom and treat them with caution. In South East Asia, fishermen are often accidentally bitten by one common species, the Beaked Sea snake (Enhydrina schistosa), while clearing nets and many subsequently die from the effects of its potent venom. The yellow-bellied sea snake is occasionally encountered washed up on shore in temperate areas such as Tasmania and New Zealand. Sea snakes are also common by-catch in the prawn industry in northern Australia, and a preliminary study to investigate the feasibility of a skin industry (for leather) has been undertaken.

Another group sea snakes are the sea kraits (Family Laticaudidae), these are a small group of strongly-banded species that are partly terrestrial and more closely related to the cobras. Unlike the true sea snakes above (Family Hydrophiidae), the species in this family must come ashore to lay eggs. They are often encountered sleeping in vegetation behind beaches or in rock crevices along the shore in tropical areas. Although inoffensive when disturbed they also have powerful venom and should be regarded as potentially deadly.