Musnad Ahmad 30:1791

Qutaybah ibn Sa'id narrated to us, from Layth, from al-Julah Abu Kathir, from al-Mughirah ibn Abi Burdah, from Abu Hurairah, that some people came to the Prophet, may Allah bless him and grant him peace, and said: "We travel far in the sea, and we do not carry with us of water except one or two water skins, because we do not find game until we go far. Shall we perform ablution with sea water?" He said: "Yes, for its dead is lawful, and its water is purifying."