The Floating City was moving slowly underwater. Even though the God of Magic Net had already reached level 13 and was able to bring the pinnacle of level 12 powers to bear, the Floating City was still unable to move over 100 kilometers per hour underwater.
If it had not been for the waters of the Oceanic Ice Block being extremely vast, the usual shallow seas would not have been able to allow something like the Floating City being submerged entirely underwater and still move.
While the Floating City was only 36 kilometers square, it nonetheless kicked up intense waves within 1000 kilometers radius while moving underwater.
Saleen's fleet had probed the north side of the Oceanic Ice Block before. No one else had been able to make it back other than the Winged Skull. That only message that returned was enough to make Saleen give up trying to send scouting fleet there.
He simply was unable to afford such losses.