The Undercaves
The game does feel puzzly in a sense.
It didn't make sense to me that the shell would kill the rocks. But I saw I could safely touch the shell, and the rocks were in my way, so I just figured these two elements might interact with each other. It comes down to this: what can I do in the level and what's in my way? Pushing around the shell was a unique thing to do, and the rocks were a unique thing blocking my way.
When you're told to swim slowly past the octopus, I didn't realize it would have to be so slow that I had to gingerly tap up.
Finally, there's a spatial puzzle in the form of routing. To make the most of your air, you'll have to plot an efficient course. The map here helps a little--if you could see the whole thing, would routing be trivial?