They are very rare and brand new to the market. I know this because I was the one that certified them.
Brand new to the market and fresh from NGC
"Cuban Waters Wreck c. 1807" genuine Spanish shipwreck 8 reale coins
This is a batch that I personally located and brought back from Cuba, with paperwork to prove the pedigree and get them certified as officially from Cuban waters. You will not see these unless they passed through my hands at some point.
Divers located the coins off 20' of water off the coast of Cuba, could not find a ship site, but chiseled these out of the coral. I have been back and there are none left. I've been able to certify only about 75 coins and they range in dates from 1780-1807 more common from mints Mexico City and scarcer from Lima Peru, New Guatemala and Bolivia.