I'm pretty sure the eternally-smoking hooves are magical actually. Handy magic, but definitely magic.
I see what they were trying to do; it matches up very well with the classic image of astral projection. That said, it matches up very poorly with going to physical places that you can physically go to.But the whole kicker here is Astral Projection. One of the most broken spells in the game.
By level 15 it's ratable, but I don't blame Inevitability for leaving it out of both nightmares' ratings. Especially since it's such an infamously busted spell. (Bright side, it gives everyone on the team basically equal benefits, so if you're wrong the cauchemar won't steal the show.)
Aquatic elf, lacedon (ghoul), merfolk, merrow (ogre), nixie, sahuagin, triton. Arguably all the low-level undead/constructs; maybe the kapoacinth (gargoyle), scrag (troll), and sea hag (hag). Definitely the dozen or so low-CR aquatic animals, if you can find a way to make them intelligent. Don't get me wrong, there's fewer in the water than on land, but not just a few.
I suppose I can get behind the idea that locaths would have radically different level adjustments in land-based and aquatic campaigns.