After defeating Volvagia, Wishiwashi-School and the Forest Master I got all 3 gems but I still cannot enter the Matsuki Castle. That's what the guards say: 

 

Volvagia: 

Wishiwashi-School: 

Don't have screenshot of defeating Forest Master. Interacted with every object mentioned on the guide but can't get in the boss room again. 

This is what the sign at entrance says: 

Screenshots of Interacting with the objects: 
1:  

2: 

3: 

4:  5:  6:  

7:  8:  

9: 

Hello @Larcan. 

 

It looks like you might have not finished the water dungeon entirely yet. This usually happens when users forget to interact with the rock in the Wishiwashi-School room (The one we can see in your screenshot.) Apart from that everything looks good, so maybe try again to interact with it in the water dungeon. 

 

Let me know if that worked. 

Have a nice day 

It worked, ty.