Amir bin Wathilah (may Allah be pleased with him) said that Abu Bakr al-Siddiq (may Allah be pleased with him) was asked about the dead (animals) of the sea, and he said: "Its water is pure and its dead (animals) are lawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 4