Sayyiduna al-Araki (may Allah be pleased with him) narrates that he asked the Prophet (peace and blessings be upon him) about sea water, and he (peace and blessings be upon him) said: Its water is pure, and its dead animals are lawful.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1080