Versions:

Minecraft version: 1.18.2
Immersive Weathering version: 3.0.3
Moonlight Lib version: 1.17.17
Forge version: 40.3.6
Supplementaries version: 1.5.18

Feature:

When underwater in cold biomes, the screen shows the same frosty vignette and freezing hearts as the powder snow does. It would be interesting if it also made the player and mobs take damage.

Additional info:

I don't know if it's intentional or a bug, but one strange thing I have noticed is that the hearts keep changing color underwater, as if they were flickering.