You might want to research the Manila Galleons. They were Spanish treasure ships that sailed from Acapulco to the Phillipines. In 1565 they discovered a return route on a north easterly course and brought Oriental treasure back to Acapulco which was brought overland to Vera Cruz and shipped back to Cuba/Spain. Some of the Manila galleons were lost/attacked along the California coast. They were much larger than the Atlantic ocean Spanish ships. Sir Robert Marx has written books on the Manila Galleons.
Sure, and if you study their history and routes, you will see that only the very early ones, of that 150-ish years (approx 2 trips per year) ever went by the coast of anything north of the Channel islands. The very early ones perhaps sighted the Pacific coast as far north as San Francisco, before heading south to Mexico. But within a short time, they were learning to eclipse that length and turn south much further out at sea. Such that they sighted the coast way south . Like somewhere @ the channel islands to Baja CA. Hence missing much of the also CA coast.
And even to the extent that it *is* true that some went missing (could have gone down in the middle of the ocean, by the way, and not on the coast), and even to the extent that some *could* have crashed on the alta CA coast (southern CA, or even central coast Ca), it STILL doesn't do md'ing any good.
Because on their return voyages from the Philippines, they were laden with trade goods. Not gold and silver (as in the ala Mel Fisher lore stuff). They had taken their $$ (gold and silver) TO the Philippines to buy the goods that were bound for the return trip to Mexico. And that would be silk, porcelains, wax, spice, etc.... Hardly the kind of stuff you'd care to find metal detecting, eh ?
There was an account of a wrecked Manila Galleon found, in the 1950s (?) along a barren remote stretch of Baja CA, south of San Diego. Must have come ashore and crashed @ the shallows. Because the beach, after erosion, was said to be littered with broken porcelain shards (woohoo) and wax globs (woohoo). Silk, spice, etc... was long gone .
While it's true that they weren't carrying gold & silver during that 150-ish yrs, the *best* there could have been a captains personal pay box. In his personal cabin.
But .... relatively speaking ........ this is all highly speculative. I mean, this is like someone musing that "the middle of Antarctica or the Sahara desert *might be a good place to detect*. Since, ya never know if some dude, 500 yrs. ago, might have been walking on just this area, and buried his goodies there.