Very good take on the "echoes" theme.
This game brings in a very interesting mechanic with the echolocation and I think it's a great concept overall with some good execution. The sound design is also great and really added to the experience.
I have to agree with other reviewers here that the submarine is very unwieldy to control, which I think is fine as a concept but it's just a little too difficult to manage, maybe just slightly increasing turn speed or something to that effect will give it that edge to make it feel right.
I never got anywhere near the 10 treasures goal so I would also recommend making that number quite a bit lower.
Great work overall!