Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.002.

Sea water’s purity is based on the following hadith: Abu Hurairah related that a man asked the Messenger of Allah, upon whom be peace, “O Messenger of Allah, we sail on the ocean and we carry only a little water. If we use it for ablution, we will have to go thirsty. May we use sea water for ablution?” Said the Messenger of Allah, upon whom be peace, “Its (the sea) water is pure and its dead (animals) are lawful (i.e., they can be eaten without any prescribed slaughtering).” This hadith is related by “the five.” At-Tirmizhi calls it hassan sahih, and al-Bukhari says it is sahih.