Running TFC version 4.0.5 beta on 1.21.1. Tested alone with only patchouli to confirm.

Minecraft water

TFC salt water. Same for all other TFC liquids.

do you have shaders / rubidium installed?

I confirmed this with only tfc and patchouli. I got distracted with other things after confirming the issue which is why the screenshots were taken with my other mods once again present. But the issue was not changed by that.

The closest I’m running to shaders is simple clouds and distant horizons and I’m using sodium.

This is known to me, it's not a shader issue.

Tracked this down to the minecraft class Camera.class#getFluidInCamera which checks for FluidTags.WATER for returning the water fog type. Since it looks like we resolved not to add our waters to the WATER tag, we need to mixin here, and I am not sure what type is most appropriate.