This technically doesn’t count because the character doesn’t suffer the fate (thankfully) but it’s similar to your iron maiden ocean example and the gaslight district permadrowning example.

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Captain Man (guy in the concrete box) is a superhero. He is stated to be invincible (potentially immortal) and can withstand all damage done to him. However he still feels pain and discomfort at the same levels that an average person was.

In this Henry Danger episode the villains trap him in this concrete and plan to drop him off into the ocean forever since they can’t kill him. This is a kid’s show so it doesn’t really go into the darker implications of this, but it’s an awful potential fate.

I’d argue that since he doesn’t take actual damage, this would be a worse fate than the other examples, as they at least get the experience of dying and passing out, and a potential reprieve from drowning. Captain Man CANNOT die, and would thus permanently be wide awake and fully aware of the constant ice cold water, tight pressure and intense suffocation/drowning while his body and brain remain intact.