Sunan Ibn Majah Book 1, Hadith 122
The Prophet was asked about seawater, and he said: "Its water is a means of purification, and its dead meat is permissible." (Hasan)
Another chain with similar wording.
"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.'"
"The Prophet went out to relieve himself and when he came back, I met him with a water skin and poured water for him. He washed his hands and his face, then he went to wash his forearms but his garment was too tight, so he brought his arms out from underneath his garment and washed them, then he wiped over his leather socks, then he led us in prayer."