Water World: Starting from Rescuing the Nine Girls of the Girl Group

The glaciers have completely melted and 95% of the land has been flooded. Only sporadic "floating island city-states", "undersea ruins" and "abandoned ports" remain. Fresh water, food, and fuel are hard currencies. The ocean is filled with mutant creatures (giant octopus, electric eel shark, ship-eating jellyfish, etc.). Human forces are divided into three categories: plundering pirates, group survivors, and the "Floating Island Alliance" that monopolizes resources.