There are 317 cubic miles of ocean, and its estimated you can get 1mg of gold from every ton of seawater.
62.4 lbs per cubic foot gives 4.6 billion tons of water per cubic mile, or 1,456 billion tons of water on the earth.
That would give you 1.456 billion grams, or 1,456,000 kg, or 1,456 metric tons of gold from the oceans if you could get it all.
In comparison, total gold mined ever is (if I got the info and math right) about 100,000 tons, and annual US production is about 256 tons.
So seawater gold is not insignificant, and equals maybe 6 times US annual gold production, but would only equal about 1.5% of total mined gold ever. And it's expensive to get to.
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It's pretty snappy for an econ book. 
