When a player uses bed ( right-clicks on the bed), a respawn point is set where the player is standing at that moment.　So, if you surround the area with obsidian or other hard blocks and fill the space with water, the player will drown forever every time he respawns.（Even though there are safe places around.）Underwater should not be selected as a respawn point unless the player has been granted water breathing or conduit power.

*P.S. The easiest way to fall into a death loop is to use the bed just before falling from a high place. This may happen more often than the above method.(At night you can end the loop by going to bed).

Translated by DeepL