The first prohibited food is the flesh of “dead animals,” that is, the animal which dies of natural causes, without being slaughtered or hunted. There are many health hazards in carrion.
But Allah has created other beings in such a way that they are able to benefit from carrion as a source of sustenance.. An exception to this rule is sea-food. The Prophet, may Allah praise him, said of the sea:
‘Its water is pure and its dead are halal (to eat).’ (Musnad)
This maybe due to the preserving factor of salt coupled with the fact that it is next to impossible to catch live fish and “slaughter” them. It may be also due to the physiology of fish itself.