Gold still has value for buying old-world relics, but on the seabed, survival relies entirely on The Drop System:
Dew-Coins: Small, magically sealed glass vials holding exactly one sip of pure, clean water. They are minted by the major factions and used as everyday pocket change.
The Blue Standard: A standard wooden gallon-jug of water is worth a hefty pouch of gold. Paying someone in actual water commands immediate respect, loyalty, and safety.