Zletovar

Poul Anderson wrote underwater action in three works:

The Merman's Children (historical fantasy);
Ensign Flandry (Dominic Flandry);
"The Game of Glory" (Dominic Flandry).

"The Horn of Time the Hunter" has aquatic human beings but is any of its action underwater? I can't remember. (Will reread, of course.)

"...the vaz-Siravo of Zletovar lived in, and between, six cities more or less regularly spaced around a circle."
-Ensign Flandry, CHAPTER SEVEN, p. 70.

Is "Zletovar" meant to be a Starkadian term or is it derived from anything Terrestrial?

The six cities are:

Tidehome;
Reefcastle;
Shellgleam;
Vault;
 Crystal;
Outlier.

The first two, in shallow water, have been bombed with stones by the Kursovikians whereas the remaining four, on the verge of the Deeps, have been unknown by land dwellers until now.

See:

Back To Starkad II
Back To Starkad III
Starkadian Mathematics
The Technic History And Some Of Its Details

Terrans call Zletovar the Sixpoint although Flandry thinks of it as "the Davidstar."