Mari is a hero known as the Holy Diver.
Mari is visiting a coastal town for the summer, studying and practicing, taking lessons for her diving certification. While she is there, people keep disappearing, vanishing on the beach at night. The beach is shut down at night, but seems fine during the day. One day, she goes down on her first solo dive, chartering a boat, diving down under ; and finds some ancient underwater ruins. She doesn’t understand, but goes to explore and finds what looks like a long piece of curved metal, surprisingly untarnished for being underwater, no rust or impurities on the surface, kind of looking like a bow but with no string or place to string a string. There is a writing on the pedestal where she finds it, saying in a weird ancient language (which she can read, unexpectedly) that this is the Ondabow. When she picks it up, something comes crawling out of the dark crevasse in the ruins, a strange amorphous thing that doesn’t make any sense as a creature, Cthulhu-looking with too-many eyes and tentacles, and no real order to its physiology, exactly the size of a deer. She starts reeling backwards to try and swim away, the bow in her left hand, and her finger snags on something, as she realizes that though there isn’t a string there, if she puts her hand where there would be a string, she can draw back and fire off what looks like a water missile, a white light that fires out and bolts through the water. She pierces the creature, which vanishes in a cloud of black inky goop. Getting back up over the water, she gets to the beach and examines the object, but can’t fire it above the water, it only works underwater. She realizes that this might be the thing that is attacking the townspeople, and decides she is going to get back into the water using her new diving certification and her new bow to defend the town from whatever threat is happening under water, under the name Holy Diver.
Nazi treasure hunt
One day, Mari is in a bar when an individual stand up and shouts that he’s heard there is a Nazi U-boat off the coast with buried sunken treasure, and that it’s free game if anybody wants to go try and get it. She goes to find the spot and dives in, and right away realizes that this ocean is insanely crowded with rival divers trying to find the treasure. She feels like a pierce straight through her heart, a cold chill, but she can’t see anything. She fires her bow, which gives her a bit of light, enough to see a Cthulhu-style alien tentacles reaching out of the submarine and grabbing divers. It becomes apparent than the Nazis were experimenting on some kind of monster. She fires her bow left and right, enough to protect herself. She swims up to the top of the water, and sees a huge rainbow in the night. She yells "shame on the night !" for doing this to her. She swims to shore and realizes she has to go back and find what’s in there.