And by the sea that is Masjur.
The majority say it refers to the oceans of the earth.
Masjur, means, the sea will be kindled with fire on the Day of Resurrection just as Allah said in another Ayah,
وَإِذَا الْبِحَارُ سُجِّرَتْ
And when the seas become as blazing fire. (81;6)
i.e., it will be kindled with fire and become a raging fire that surrounds the people in the gathering area,
this was reported by Sa`id bin Al-Musayyib from Ali bin Abi Talib.
It was also reported from Ibn Abbas, and it is the view of Sa`id bin Jubayr, Mujahid, Abdullah bin Ubayd bin Umayr and others.
Qatadah said,
"Masjur is the `filled sea."'
Ibn Jarir preferred this explanation saying, "The sea is not lit with fire now, so it is filled."
Allah's statement;
إِنَّ عَذَابَ رَبِّكَ لَوَاقِعٌ