Is it permissible for a Muslim to perform Ghusl (ritual bath) with sea water in order to perform the prayer even when he is obliged to do so?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Yes, it is permissible for a Muslim to perform Ghusl (ritual bath) or ablution even if he is not obliged to do so because sea water is pure in itself and it is a purifier, i.e. it purifies others [or other things], as the Prophet said (about the sea): "Its water is pure, and its dead fish is lawful to eat.”  [Abu Daawood and An-Nasaa'ee]  In addition to this, the term water in general includes sea water as well. So this is included in the Saying of Allaah (which means): {…and you find no water, then perform Tayammum (seek clean earth) and wipe over your faces and your hands [with it].}[Quran 4:43].