Instructions
Accept the quest from Renn Mcgill in Dustwallow Marsh. Upon accepting the quest, you will receive Renn's Supplies, a bag located in your inventory.
Open your inventory and open Renn's Supplies to receive the Salvage Kit and Repaired Diving Gear. Equip the repaired diving gear to breathe underwater.
Enter the ocean and swim until you see floating boxes and wooden planks. Once you reach the floating items, open your inventory and right-click the Salvage Kit. Eventually, you'll receive a Salvaged Strongbox, although it may take several attempts to use the Salvage Kit on various floating debris.
Return to Renn Mcgill once you have acquired one Salvaged Strongbox to turn in the quest and choose your reward.