Seeweegia is the "strange and beautiful undersea kingdom" of jellyfish people, to be found in the Nonestic Ocean.
Seeweegia is a brightly-colored growth of seaweed in the southern Nonestic Ocean, past the Sea Forest. It is the home of transparent jellyfish men and women with round, square, and triangular faces. The people live in caves and grottoes in the sea bed, which is covered with bright shells, polished stones and all manner of sparkling deep-sea jewels. (Captain Salt in Oz)
Though one might logically expect the name of the place to be spelled "Seaweegia," as in "sea weed," printed sources agree on the spelling of "Seeweegia."