Julie is a twelve-year-old diver who lives with her mother in Atlanta during the school year and spends summers helping her father with his dive business. She is action-oriented and highly knowledgeable about diving safety protocols. Her concern over her father's financial struggles and declining health drives her to act maturely. She frequently takes on adult responsibilities at his shop to keep the business operational.

Shane is a tall, athletic twelve-year-old boy who attends boarding school and comes from a wealthy family. He wears popular brand-name clothing and initially shares his father's disregard for safety rules. Underneath his confident exterior, he harbors insecurities stemming from a distant dynamic with his workaholic father.

Gibson is a passionate diving guide with unkempt gray hair and sun-faded clothing who owns a struggling dive shop in Gulf Shores. He suffers from diabetes and frequently neglects his physical health in pursuit of his business goals. His past obsession with locating a secret artificial reef contributed to his divorce and left him financially strained.

Hank is a wealthy attorney who hires Gibson Sims for an exclusive, highly priced dive at an undisclosed location. He is arrogant and consistently disregards standard safety protocols in favor of spearfishing. He uses his financial status to assert dominance over others and pushes his son to document secret coordinates for future exploitation.

Mrs. Sims is Julie's mother, a hardworking lawyer employed at an Atlanta firm. Her demanding career provides financial stability but leaves her exhausted and limits her free time. She divorced her husband over his obsession with a sunken reef, though she maintains a close, protective bond with her daughter.

Dr. Malzon is a deceased figure from three years prior to the main events of the story. He hired Gibson Sims to create an artificial reef using abandoned military tanks. His sudden death shortly after receiving the secret coordinates to the reef set off a chain of events that financially impacted the Sims family.