I just played the game! I tried my best to find the treasures, I believe I did find one while digging close to the X mark on the map, since when I heard a different sound while I was digging around that area.
I couldn't manage to craft anything, I should have experimented more but at the moment I wasn't able to do so. Unfortunately my playthrough stopped when I clipped through a block, I tried swimming around the island and then when I wanted to get back, I somehow managed myself to get stuck by the shore, so I had to restart all over again.
It is an interesting game though! I would have liked to try it in multiplayer.