Sunan Ibn Majah Book 1, Hadith 120
He heard Abu Hurairah say: "A man came to the Messenger of Allah and said: "O Messenger of Allah, we travel by sea and carry a small amount of water with us. If we use it for ablution, we will become thirsty. Can we perform ablution with seawater?' The Messenger of Allah said: 'Its water is a means of purification, ad its dead meat is permissible. (i.e. the fish found dead in the sea).'"
On the night of the Jinn the Messenger of Allah said to Ibn Mas'ud: "Do you have water?" He said: "No, only some Nabidh in a large water skin." The Messenger of Allah said: "Good dates and pure water." (i.e. there is no harm from the mixing of the two.) Pour it for me." He said: "So I performed ablution with it."
"I was fishing and I had a vessel with me in which I kept water, and I used seawater for ablution. I mentioned that to the Messenger of Allah and he said: 'Its water is a means of purification, and its dead meat is permissible.'"