It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) was asked about the sea. He (peace be upon him) said: "Its water is pure and its dead (animals) are lawful."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب ذكر الماء الذي لا ينجس، والذي ينجس إذا خالطته نجاسة / 112