Well, the monstrous variety depicted in ancient sailors' lore may not be much on the minds of modern mariners.
Venomous sea snakes mostly inhabit the waters of Australia, Indonesia, Southeast Asia, and India.
Some, however, are much more wide-ranging and are found from southern Siberia to the coast of New Zealand and Tasmania, from Cape of Good Hope to Central American Pacific waters. Their presence presents a potential danger to swimmers and divers, so it's worth while to have some knowledge of their habits and habitats and of medical treatments available if a crewmember is bitten. There are possibly more sea snakes in the world than any other kind of snake, and almost all are venomous. Sea snakes are air-breathing animals. But evolution has provided them with some advantages over their land-bound kin.