Strange key & Bart, Pirate's grave
 

I am in Freedom Islands and bought the strange key for 20k.
I wonder where the pirate's grave is? Thanks.

Edited:
Never mind, found it.
His grave is the big coffin at the end the little cemetary area on the Western Islands. After freeing him you can fight him on an isolated island on the east side of Eastern Island (and then dig for some random treasure).