Hey there 666, do you have some sort of conclusive lead on where a "400 oz" (25 pound!) bar exists ? I mean, to say specifically that's in 3 ft. depth of water, @ 20 ft. from shore, sounds like you have a location pinned down, eh ? That's over half a million in gold melt value. Are you sure you're not on some urban legend tall tale chase ?
But in any case, it's sort of as Jason says: Sand shift around all the time (assuming you have a bottom that includes sand? Versus 100% always corral hard??) . I have scuba dived and chased aluminum cans to 2 ft. deep, that were crisp fresh losses. Doh! *Obviously* it's simply a case of the sand moving around and covering and un-covering stuff all the time.
For example: You know how the inter-tidal zone has sand that changes around with the seasons all the time, right ? Beaches erode and re-fill with each rough surf, swell, tides, etc..... SO TOO is it with the under-sea world. The sand is not "constant". There are "dunes" underwater every bit as much as there are "dunes" above the water.
So there's no answer to your question. But for an object of that value , and seeing as how you have the exact location, why is the depth sinkage an issue ? For that price you can get heavy equipment (3 ft. wont' even reach the hubcaps) and just dig that puppy out with an excavator. His bucket swings around, and dumps 4 yards of spoils at a time on the beach . Or dredges or sand pump fans, etc... Afterall, if your investment at such measures is $10k to $100k, what do you care ? You're about to be 500k richer, which more than pays for the equipment.