Musannaf Ibn Abi Shayba 1:1394

Abu Dawud al-Tayalisi narrated to us, from Hisham, from Qatadah, from Abu Ayyub, from Abdullah ibn Amr, who said: The water of the sea does not suffice for ablution nor for major ritual impurity (janabah). Indeed, beneath the sea is fire, then water, then fire.