Abu Hurayrah (may Allah be pleased with him) reported: A man asked the Prophet (may Allah's peace and blessings be upon him): "O Messenger of Allah, we travel by sea and carry with us a little water. If we made ablution with it we would become thirsty. Can we make ablution with the sea water?" The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Its water is pure and its dead animals are lawful (to eat). "
شرح الحديث :
In this Hadīth, the Messenger of Allah (may Allah's peace and blessings be upon him) clarified that sea water is pure and thus it is permissible to use it for purification, and that it is lawful to eat dead sea creatures, like dead fish and so forth.