There is a science experiment in the ship that involves three rocks. You need to get them all to resonate on the correct level. The device is switched on from the lever in the left.
This is the low-spoiler guide for "Three black rocks experiment" puzzle in Call of the Sea. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.