Abu Dawud Book 1, Hadith Number 83.

Chapter : Performing ablution with sea water.

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.