Abdullah bin al-Mughirah narrates that a man from Banu Mudlij told him that they set out on a sea journey to hunt and took water with them for drinking. The time of prayer came upon them while they were at sea. They mentioned this to the Prophet (peace and blessings be upon him) and said: If we were to perform ablution with our water, we would be left thirsty, and if we were to perform ablution with sea water, we would feel uneasy about it. The Prophet (peace and blessings be upon him) said: Its (that is, the sea's) water is pure (or: its water is a means of attaining purification), and its dead animals are lawful.
Imam Ahmad recorded this narration, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1078