Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that a man asked the Messenger of Allah (peace and blessings be upon him): "O Messenger of Allah (peace and blessings be upon him), we travel by sea and keep a little water with us. If we perform ablution with it, we are left thirsty. May we perform ablution with seawater?" He (peace and blessings be upon him) said: "Its water is pure and its dead are lawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيد والذبائح / 18965