The Water Planet is actually a moon of the gas giant remnant Cohen 136 (Iridia) V. It was colonized by Biran and Humans before the fall of the Empire.
Though it is definitely a moon, the inhabitants tend to insist on the name being The Water Planet.
Physical Planetography
The moon is very similar to the Biran homeworld; Biran scientists expressed some initial surprise that it did not already have life.