Talk to a 'circle' dolphin- you’ll find him eventually, he’ll activate something, and the 3 of you can now push the crate of rocks.
Go through that tunnel, and each time press the buttons (or sonar them) and go through, the rocks are essential.
You need to be fast as well for the last bit of crushers.
When you’re passed the crushers, push all the rocks so that it makes weight to open the ground.
Go down to the next level.
