I used to hunt, and I had a waterskin in which I would carry water. One time I performed ablution with seawater, and I mentioned that to the Messenger of Allah ﷺ. He said, "Its water is pure, and what dies in it is lawful to eat."
حدثنا سهل بن أبي سهل حدثنا يحيى بن بكير حدثني الليث بن سعد عن جعفر بن ربيعة عن بكر بن سوادة عن مسلم بن مخشي
كنت أصيد وكانت لي قربة أجعل فيها ماء وإني توضأت بماء البحر فذكرت ذلك لرسول الله ﷺ فقال هو الطهور ماؤه الحل ميتته
[Its chain of transmission is daif] Muslim bin Makhshi is obscure and only Ibn Hibban has deemed him trustworthy. As for Ibn Firasi, I have not found anyone who deemed him trustworthy, but the previous hadith (386) makes this one unnecessary.