Chapter 11: Orishan
These creatures actually may not have evolved that indestructible shell for defensive purposes. If they did, they got a bonus: water compaction. With that rigid armour, they can probably push water to levels that should turn it to ice! If they could do that, they would be able to hold tidal amounts of water squished in their shells. They would also be able to use pressure differences to pull water in, creating the water whips. They could also use this to pull in air and filter out the water, keeping it in their shells, and return the air to where it belongs. It is possible they have two layers of armour, or that the organs are adapted to that pressure. I believe they can also breathe via the water in their shell. The fact that they are bipeds is probably to make it so that they can walk and swim in the ocean.