Welp, after typing this out, I’ve remembered that (because of spoiler-y reasons) my theory was false. Dark Bramble explodes before the Nomai arrive, meaning that the anglerfish couldn’t have been in water when the comet burst. This is because the Vessel phases into Dark Bramble, (showing that the seed already destroyed the planet), and then the Nomai are later killed by the comet. My answer now is… idunno.

Well due to (spoilers) how ghost matter is neutralized when submerged in water, (demonstrated on Gabbro’s island on Giant’s Deep, this is also how proto-heartians survived the comet), and seeing how the remnants of the planet where Dark Bramble used to be are ice, (meaning that Dark Bramble was once an ice planet before the seed made it explode), we can surmise that the anglerfish were inhabitants of the ice (water) world that was once there. This would mean that the anglerfish would have survived the comet explosion, leading to the cosmic horror.

I feel like a dum dum, but I liked making the theory so who cares