I am not being silly. You are simply seeing the conundrum in a more obvious way and realizing it obviously can't be right. You're almost there. How is your way simpler? A rigid tube requires no moving parts and no energy input. If your ideas about energy from pressure were sound, why would the tube not work? That is a question left for you. Once you solve it, you will see why no system can extract ocean-depth water pressure to do work. And that, if compressed air is the goal, it can be made at less energy cost with resources we already have. Ocean water pressure is no more a source of harvestable energy than rocks at the bottom of a cliff. They are as low as they can get. The only way to extract any energy from them is to first put energy in to raise them.