Here's my theory - there are a lot more fish in the sea than there are geese in the air. Ergo, a fish that coughs up gold jewelry is much better news than fables recounting a single goose who squeezes out 24k ovum.
Just from the sheer volume of wise-guys wrapped in chains and dumped off of watercraft into the murky deep should lend credence to the enormous wealth potential of our swimming oceanic friends.
"Sleeping with the fishes" could also mean "shopping with the fishes", if you catch the way I've drifted.
This could take the place of those shady "sell us your gold by mailing it to us" commercials. Instead, by some carp and tuna and teach them how to scavenge for bling.
Fish Coughs Up Gold Watch , via Neatorama