Not sure if this is caused by Lumina or Tweaks, but lighting is very buggy underwater.

Happens with or without other mods (besides Lumina and FalseTweaks).

does this happen without FalseTweaks too?

With Lumina (On rc9):

Yes, seems to be a Lumina issue.

(Removed FT too to make sure.)

Was that world ever opened using RPLE?

The underwater corals are designed to have a colored glow on them with that mod, but removing RPLE breaks it like this.

I never used RPLE.

Fixed in latest RPLE commit, we're open to pull requests if anyone wants to fix it in standalone Lumina.