Short answer
Only when someone dies on a ship, land is too far to reach before the body would change, and there is no way to keep it. Then the body is washed, shrouded and prayed over, weighted, and lowered into the sea. Otherwise burial in the earth is required, and today ships can usually keep the body until port.
Burial in the earth is fard kifayah. If a person dies at sea and land is so far that the body would decay before reaching it, the body is washed, shrouded and prayed over, then lowered into the sea, weighted so it sinks. If land is near, or the body can be preserved, it is taken to land and buried.
The four schools agree on this.
Explanation
Allah made the earth the resting place of the human body: "Then He causes his death and provides a grave for him." So the normal rule is clear. A Muslim is buried in the ground, and scattering ashes or a chosen sea burial is not allowed; see cremation.
The exception. Classical scholars of all four schools discussed people who died on long sea voyages, where a body could not be kept for days or weeks. In that case the sea becomes the grave out of necessity:
- Wash the body and shroud it as usual.
- Pray the janazah on board.
- Weight the body (Hanafi and Hanbali scholars mention this) so it sinks and is not left floating, and lower it gently into the sea, facing the qiblah if possible.
Some Shafi'i scholars preferred placing the body between planks so it might reach a shore and be buried by Muslims there, if the coast was near.
Today. Cruise ships, cargo ships and navy vessels usually have cold storage and can bring the body to port, and many countries require this. Where that is possible, it is required, and the body is buried on land. If a sailor dies in a remote area with no way to keep the body, the old ruling applies. For a body that is lost at sea and never found, the janazah is not prayed in the Hanafi school, but the family makes dua, and inheritance and the widow's iddah follow once the death is confirmed.
The evidence
Quran, Surah Abasa 80:21
ثُمَّ أَمَاتَهُۥ فَأَقْبَرَهُۥ ﴿٢١﴾
Then He causes his death and provides a grave for him.
The grave in the earth is the normal resting place.
Quran, Surah Taa-Haa 20:55
۞ مِنْهَا خَلَقْنَٰكُمْ وَفِيهَا نُعِيدُكُمْ وَمِنْهَا نُخْرِجُكُمْ تَارَةً أُخْرَىٰ ﴿٥٥﴾
From the earth We created you, and into it We will return you, and from it We will extract you another time.
The body returns to the earth.
Where the ruling is recorded
- al-Fatawa al-Hindiyya, Kitab al-Salah, chapter on the jana'iz, section on burial. One who dies on a ship far from land is washed, prayed over and lowered into the sea.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, Bab Salat al-Janazah, burial. Weighting the body if land is far.
- al-Mughni (Ibn Qudama), Kitab al-Jana'iz, one who dies at sea. The Hanbali ruling, also with weighting.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.