Ok, gold was made illegal for private ownership (there were exemptions for jewelers and dentists) in the US in 1933, but I don't know the history of gold coins for other countries, nor when gold coins were out of circulation enough that you'd been unlikely to find them in a sunken ship.
I'm going to assume that any silkie who was interested in gold would be tracking storms and ships so as to be able to follow a sinking ship down-- I think they'd be hard to find otherwise without tech. How deep can a seal dive? Google [seals depth -navy +mammal], you're my pal. Elephant seals dive 700 meters, harp seals (described as not as strong divers as other seals) dive 370 meters.
But what species is the typical silkie, and how deep is the typical shipwreck?
If gold is valuable to silkies, do they have an economy? What might a silkie trade to another silkie for gold? I will assume that the ocean is big enough that a silkie can just hide gold rather than needing institutions (family help?) to keep their gold from being stolen. The ocean seems safer than the silkie trying to hide gold on land.
It's all fandom's fault. I didn't used to care about world-building.
Here's a good version if you'd rather the fine old eerie stuff: