Buthos (βυθός) refers to the depths — particularly the deep places of the sea, the bottom. In its single NT occurrence (2 Corinthians 11:25), Paul lists spending "a night and a day... in the deep" among his sufferings for the gospel, likely referring to time adrift at sea after a shipwreck.
Paul's catalog of sufferings in 2 Corinthians 11 — including time in the buthos — is a stunning inversion of triumphalism. The apostle was not protected from the depths; he was preserved through them. This is the theology of 2 Corinthians 1:9-10: "we felt that we had received the sentence of death. But that was to make us rely not on ourselves but on God who raises the dead." The deep cannot swallow those held by the God of the depths.