Final answer:
The narrator's attention in the big blue ocean is caught by a mysterious creature, which is described to leave a phosphorescent trail and identified as a large shark.
Step-by-step explanation:
What caught the narrator's attention deep down in the big blue ocean was a mysterious creature. The passages provided describe an encounter with a presence in the ocean that leaves a gleaming trail of phosphorescence, speeds through the water like a shadow with a long sparkling streak, and is accompanied by the sound of a whirring fin. This creature is recognized as a shark by the characters in one of the passages, noting its significant size and the interest it showed in their vicinity.