It's estimated that over one third of all gold mined in America lies
in less than 30 ft of water near shore due to shipwreck. That's about
$2 billion in potential recoveries not counting other treasures that
can be found in wrecks in the sea.
More likely than not, you're more interested in finding treasure on the beach. Luckily, storms and gales literally redeposite treasures lost at sea onto the beach.
When you've finally reached your destination at the beach, find the high tide mark and the low tide mark and imagine the median line between them. You'll want to conduct your search within 10 ft of that line on each side. Get out your shovels and buckets, you're gonna be around for a long time unless you've got a metal detector. You're not as likely to find gold coins with a metal detector but you can get rich from change lost out of peoples swim trunks.
If you do have a metal detector start your search with a zig zag pattern within your 20 ft zone up the beach. When you're coming back, do the same thing; you'll probably cover the areas you missed going up.