Do you mean like drowned in the sea? Or do you mean if someone dies whilst at sea?

Well if someone has drowned at sea, and the body has been found and retrieved then we wash them and pray over them as usual and Insha'allah this person who drowned is a Shaheed.
If we are not able to retrieve the body from the sea then we just pray over them Salaatul Ghaa'ib.

If someone dies whilst at sea (on a ship or boat or ferry or yacht or canoe or raft etc etc. ) Then according to the Ulama we wash the body and shroud it pray Salaatul Janaazah over it and then tie something heavy to it and drop it into the sea. ( to get eaten by sharks )

But if those people at sea are able to reach land in a short period of time, then they should wait until they reach land and perform the burial on land....otherwise the body is to be dropped into the water like I mentioned above.

Wallahu A3lam.

Okay next question :

Is it Sunnah to walk barefooted?

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Umm-Sulaim, if you can Give us the fawaa'id about the three different types of Shuhadaa' I'd be very grateful ;)