Aika R-16 exists purely to deliver fanservice with zero apology, and it moves fast enough that you don't have time to notice the plot is basically a framework for the next scene. The character chemistry between Aika and Erika is genuine, which somehow makes the entire thing work despite the fact that nothing here makes sense. The animation is sharper than it has any right to be for an OVA this committed to its own shamelessness.
Aika just earned her salvage license, and she's ready to prove she belongs in the underwater treasure hunting business. Problem is: nobody wants to hire a hotheaded teenager, no matter how capable. So she takes matters into her own hands, advertising her services around school until she catches the attention of Erika, the wealthy and charming leader of the treasure hunting club. Erika's offer seems perfect. One salvage job, and Aika's career launches. But when Erika reveals the scope of the operation, complete with a state-of-the-art submarine and a dangerous destination, Aika realizes she's in way over her head. What should have been a straightforward gig turns into something far more complicated and perilous than she bargained for. Between her reckless confidence, Erika's mysterious motives, and the chaos of deep-sea salvaging, Aika learns that getting the job is only half the battle. This is action-packed comedy for anyone who enjoys watching capable characters get humbled by circumstances they didn't fully understand.