Detailed guide with pictures and texts on Datide Reef:

Go to Tidal Reef to get the Coral Saber. At first, you can use your rain boots to move forward smoothly until you reach a place with two statues. First push the stone statue as shown in the picture (the iron box is at the back, use rain boots to jump over it to push it back), and then use a bomb to detonate it.

Then use the iron box as a foothold and push the statue face to face to open the iron door.

Enter through the iron gate and reach a dead end. There is a stone tablet reminding you that a widow is crying. Place a bomb in the picture and explode it to get a master key!

After getting the key, return to the high platform where you just pushed the statue, jump down from the back, and open the door.

Trigger the plot and automatically get the Coral Saber! After getting the Coral Saber, go back and there will be a green treasure chest. Open it and you can get a Heart Piece!