Short answer: ocean is very big, submersible is very small.
Long answer: Imagine going out on a football pitch with a white paint marker. Randomly walk around and pick a single blade of grass to paint white. Now grab someone that didn't observe you and tell them to find the painted blade of grass. Also tell them they have a limited time, and that you might have pushed the blade of grass doen into the ground.
Short answer: ocean is very big, submersible is very small.
Long answer: Imagine going out on a football pitch with a white paint marker. Randomly walk around and pick a single blade of grass to paint white. Now grab someone that didn't observe you and tell them to find the painted blade of grass. Also tell them they have a limited time, and that you might have pushed the blade of grass doen into the ground.