Aeolius
Adventurer
Treasure may be similarly altered, as many undersea races value rare corals, pearls, and shells far more than coins and gemstones.
To be fair, one does need to account for the occasional mermaid who hoards thing-a-ma-bobs and snarfblatts in her hidden grotto.

Coins, being somewhat rare, are not by default valueless. Many undersea races see the value of metal for weapons and utilitarian objects. A few might even see the beauty in golden jewelry forged by drylanders. Fewer still might appreciate the rarity of coins minted by "Those Above".
I look at it this way. I have a wallet filled with paper money. I might also have some Chinese yuan in there. The dollar bills will spend more readily, in my neighborhood.