There are several options.
It is possible to understand the term as referring to the underworld as the home of the devil and his minions (Isaiah 27:1; Revelation 13:1) and so the realm over which satanic forces rule. This understanding adds to the third option below a level of moral aversion, that is, a place of evil (Psalm 107:6) and so of suffering. In this reading, the fact that the
sea
must give up its dead implies that its master is defeated (Psalm 74:13; Isaiah 27:1).We could take the word literally and so understand it to refer to those who were buried at sea. The problem with this interpretation is that we are left wondering what happens to those who were buried on land or cremated.
We could also understand the word
sea
to refer to the underworld (recall Exodus 20:4:the water under the earth
). The Old Testament pictured the underworld as the assembly place for the deceased (Genesis 37:35; Numbers 16:33; 1 Kings 2:6, 1 Kings 2:9; Job 7:9; Psalm 30:3; Psalm 55:15, etc.), even referencing its water in Job 26:5. Then the idea would be that at the final judgment this place is emptied of its occupants. The problem with this interpretation is thatsea
then becomes synonymous withHades.
Of these three options, the first is the most persuasive for the present passage.
13 And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done.