Shipwrecks can spawn a box of valuable items.(Gems, Ores, Gold / Silver) which can be sold for Krypto at a trader.
Look for the above icon on the coastline of the map(zoom in) to locate a shipwreck.
Wetsuit and Rebreather are recommended for deep sea diving, oxygen is a limiting factor and you can die quickly without a Rebreather.
Keep a lookout for Sharks, they will be lurking in the deep.