A man asked the Messenger of Allah 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 purifying, and its dead creatures are lawful to eat."
أخبرنا قتيبة عن مالك عن صفوان بن سليم عن سعيد بن سلمة أن المغيرة بن أبي بردة من بني عبد الدار أخبره أنه سمع
سأل رجل رسول الله ﷺ فقال يا رسول الله إنا نركب البحر ونحمل معنا القليل من الماء فإن توضأنا به عطشنا أفنتوضأ من ماء البحر فقال رسول الله ﷺ هو الطهور ماؤه الحل ميتته
Its isnad is sahih. Tirmidhi narrated it in Kitab Tahara, Bab ma Jaa fi Maa al-Bahr Annahu Tahur, hadith (69), from Qutayba, and Abu Dawud, in Kitab Tahara, Bab al-Wudu bi Maa al-Bahr, hadith (83), and Ibn Majah, in Kitab Tahara, Bab al-Wudu bi Maa al-Bahr, hadith (386/3246), all from the hadith of Malik through this chain. It is also found in the Muwatta (Yahya's recension), 1/22, and in Kubra of Nasai, hadith (58). Tirmidhi said, hasan sahih, and Bukhari, Ibn Khuzayma, Ibn Hibban, and others graded it sahih.