Topic: Chests at The Great Sea

Hello. I just finished playing Paper Mario Origami King. Now, after I’ve collected all the treasure from the sea using the sea chart with the red X, is it possible that I can come back to the sea anytime I want and dive into the areas with gray X that may have another chest. I know that I already collected all the collectibles, but would probably assume that chests and treasure that I find will only contain bag of coins or power ups. Do chests respawn/reappear in this game or at the sea, or are all the chests inside the game can be opened only one time and that’s it?

Is Origami King hard? That ring based gameplay just looks so confusing.