The Giant Oyster is very tiny! Only one square lol. It drops 400 coins on a 2hr timer.
The Siren Statue also has a 2hr timer, but only drops a few coins. Have not seen a black flag for anything else.
The Ocean Chest is deco. Good thing about the Jelly Bloom is for coins and that it is deco, rather than the choppable ones I have left over from the event.
Would have loved more items as my Ocean area is my favorite part of my queendom!