Digging through 0E material, for reasons, and stumbled across this section which (apparently) talks about aquatic variants of other monsters:
As near I can tell, just about nothing has been done with this idea since in official material. There were aquatic gnome and kobold variants in the 3.5 Unearthed Arcana, but that seems to be it.
GNOMES: Live in air-enclosed subterranean cities connected to the surface by tunnels.
KOBOLDS: Live in air-enclosed cave complexes connected to the surface by tunnels.
As near I can tell, just about nothing has been done with this idea since in official material. There were aquatic gnome and kobold variants in the 3.5 Unearthed Arcana, but that seems to be it.