"Returning to the Ruins Profound Dark Circulation of Internal Energy," this technique was originally developed to adapt monks for shallow sea combat, equipped for operating in deep water, which a Nascent Soul Expert could learn without much delay.
This water evasion technique only grew more efficient the deeper one dived, swimming faster, so Li Fan and four others went directly into the sea under cover of night, skimming along the seabed like shuttle fish with the aid of talismans from Master Chen, noiselessly evading the surveillance of Xuanmen and Immortal Palace monks above Flame Island.
Having computed the location of the spiritual array, the five easily followed the ley lines to find the spirit cave where the sword forge was located.