Abdullah bin Amr (may Allah be pleased with them both) said: It is not permissible to perform ablution (wudu) or the ritual bath for major impurity (ghusl) with sea water. Indeed, beneath the sea is fire, then water, then fire, until he counted seven seas and seven fires.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8665