Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 83
Narated By Abu Hurayrah : A man asked the Apostle of Allah (pbuh): Messenger of Allah, we travel on the sea and take a small quantity of water with us. If we use this for ablution, we would suffer from thirst. Can we perform ablution with sea water? The Messenger (pbuh) replied: Its water is pure and what dies in it is lawful food.