A man asked the Prophet ﷺ, "Messenger of Allah, we travel by sea and carry only a small amount of water with us. If we use it for ablution, we go thirsty. May we perform ablution with seawater?"The Messenger of Allah ﷺ replied, "Its water is pure, and its dead creatures are lawful to eat."
حدثنا عبد الله بن مسلمة عن مالك عن صفوان بن سليم عن سعيد بن سلمة من آل ابن الأزرق أن المغيرة بن أبي بردة وهو من بني عبد الدار أخبره أنه سمع
سأل رجل النبي ﷺ فقال يا رسول الله إنا نركب البحر ونحمل معنا القليل من الماء فإن توضأنا به عطشنا أفنتوضأ بماء البحر فقال رسول الله ﷺ هو الطهور ماؤه الحل ميتته
[Its isnad is sahih] Reported by Tirmidhi, Purification, chapter on what has been said about the water of the sea being pure, no. 69, from the hadith of Malik through this chain, and it is in the Muwatta (Yahya's version): 1/22. Nasai also reported it, no. 59, and Ibn Majah, nos. 386, 3246. Tirmidhi said, this is a hasan sahih hadith, and Ibn Khuzayma graded it sahih, no. 111, as did Ibn Hibban (Mawarid): 119.