Bring the Tool Kit and Damaged Diving Gear to Renn McGill in the Dustwallow Marsh.
There's plenty of wreckage strewn across the bottom of the water from here to Alcaz, and the Defias have already got divers out there looking around. With the help of some of their gear, we can get you suited up and ready to join the hunt, but you'll have to do some creative "borrowing" first.
Search this island for a crate of diving gear and something to repair it with. When you've got them, bring them to me and I'll have your gear ready in no time!