Musnad Ahmad 1018:35

Yazid narrated to us, Yahya informed us, from Abdullah ibn al-Mughirah ibn Abi Burdah al-Kinani, that he informed him that some of the Banu Mudlij informed him that they used to ride rafts on the sea for fishing, and they would carry water with them for drinking. The prayer would overtake them while they were at sea, and they mentioned that to the Prophet, peace and blessings of Allah be upon him. They said: "If we perform ablution with our water, we will become thirsty, and if we perform ablution with the water of the sea, we find [aversion] within ourselves." He said to them: "Its water is purifying, and its dead [creatures] are lawful."