Time for stupid Dio's coffin theories! I figured because some aspects of the Steel Ball Run universe reflect the original universe, especially fates of characters, perhaps something like what happened between Diego and Lucy also occurred after the end of Phantom Blood.
Erina clutched the orphaned baby, crying into its wrappings. Tears streamed down her face and her cheeks were blotched red from her wretched sobbing. The ship had exploded into a horrible fire, red against the nighttime sky and carnivorous and brilliantly bright. Erina didn't want to accept this fate, but it lay before her, and all she was left with was the baby in her arms and another unborn in her belly. The coffin she used as a makeshift life raft was something only a madman like Dio would've picked, fantastic and gaudy, inscribed with that monster's name in beautiful script. It rocked heavily on the waves, and she felt sick.
Sick and so very very tired.
Like a remnant of a nightmare that refused to die, a torn hand reached out from the dark unsearchable waves and grabbed the side of the coffin. Erina stared at that hand with her worn eyes. Red and burnt and covered in peeling skin like its owner had crawled through the inferno of hell. A disgustingly hopeful thought crossed her mind at that moment, even though she knew there was no way it was true. "Jona….than?"
She whispered like saying it would bring him to life. For a moment she pictured that he had survived those wounds—normally fatal but if it was him than maybe. Maybe. It was a pathetic hope, and even as she knew this she still desperately wished for him. All her life she'd lived pampered and sheltered, far form the true horrors of this world. As a doctor she'd never been assigned cases where the patient's life was at risk, perhaps it was a move of the other doctors to protect her, or perhaps she avoided them herself subconsciously. It's not that Erina would've refused to help them if she saw them before her, quite the opposite, but Erina's seen little suffering in her life.
With great strain a huge figure pulled itself from the dark ocean; burnt hands gave way to a torn suit barely holding together, limbs hanging at unnatural angles, and hair that was mostly lost to the fire. The few strands that remained were blond, plastered to his scarred head and matted with blood. Erina was speechless as she stared at the horribly vibrant scar on his neck. She blinked, and the spare tears ran clear of her eyes, giving her unclouded vision of that immortal diver.
"Dio."
He glared at her, his face shadowed and nearly unrecognizably burnt. He stood at the foot on the coffin in an unnatural slouch that Dio would have never held before. In one of his hands he held a cloth sack, but other than that he was unarmed. Not that he needed anything to defend himself with.
The baby started to cry.
Dio carefully lowered himself to crooked position seated at the far end of the coffin. It wasn't as far as Erina would've liked it to be. Not even close. She didn't take her eyes off Dio once, as she held the baby close to her and looked at him with the eyes of a cornered deer towards a wolf pack. She was trembling slightly, still crying but to frightened for the tears to fall.
The baby's cries turned into hollers, and Dio glowered at it. "Turn it off or I'll throw it into the ocean," he commanded Erina. She glanced at the gaping waters fearfully and quickly, but enough for Dio to spot, "If that doesn't suit you I can always eat it. Or make you eat it."
He was a shadowy figure in Erina's eyes; she couldn't even see his face in the night, if he even had face left at all.
"Or it eat you," Dio added impatiently. There was a growl to his voice that hadn't been there in their youth.
Erina cradled the baby and whispered to it softly, to Dio's growing annoyance. She gave the baby false coos of comfort and unsupported promises spoken frantically, trying to calm it before Dio's fury grew. Even in his current state he was a vampire that tread towards immortality and she was a normal human who'd never known true danger before, so if he chose to attack her there was little she could possibly do to protect the child.
The baby quieted, but the coffin didn't, bouncing and jostling at every wave it hit, spraying seawater over the strange occupants. Another sound that could be heard was Dio's skin stretching over his burnt body as it repaired itself from the fire, squelching and ripping and the occasional crack as bones worked their way back into place and flesh wove together.
Erina saw the sack that Dio had carried onto the ship sitting between the two of them. Overcome with curiosity, she leaned forwards and grabbed its hem. Dio clearly noticed what she was up to, but made no move to stop her. That should have been her first warning. Carefully, she opened the mouth of the sack and stared into the darkness that lay within. At first she looked curious if scared, but then her vision fixed on the contents of the bag and her eyes widened and irises diluted as large as dark moons. Her face stretched and the circles under her eyes darkened. She paled quickly and the baby fell silent sensing her horror.
Erina closed the lid of the sack with trembling hands. "You've grown into a horribly pitiable creature," she said. It wasn't a smart thing to say, but after seeing that all other words had flown her mouth.
Dio snarled. "Pitiable?" His entire damaged face twisted into an expression like a hungry wolf.
"I've thought you a horrible human being since the beginning, but you aren't even that anymore." Dio was shaking in fury, but Erina continued on, "It's not because you became a vampire that you can't be considered human, it's because of that. Never have I met a human carrying something like that around. Never has a human threatened a baby like you have, never has anyone qualified as a human taken so many lives."
Dio laughed, hollow and vicious, like the final gasps of a dying man, "Yes, I'm a monster. A being who has far surpassed humanity! That was the last remnant of my humanity, the bridge to my new existence." Dio leaned forwards, and his eyes which were now visible to Erina started to glow. It was the same attack he had used against Jonathan.
Still, even as those horrible memories flashed before Erina's eyes, she talked like she remained calm, "Even if you kill me, what good will it do? The sun will soon rise." The first signs of daybreak stretched across the sky in light blue lifelines. Like magic, Dio's fury shifted from Erina to the sun which imprisoned him and the darkness he could never escape. There were blisters already starting to appear on Dio's newly healed skin. Much longer and he would return to dust. "Why don't you go inside the coffin?" Erina asked shakily. "That way, you'd be safe from the sunlight."
Dio looked at her dubiously and didn't move from where he was. Nevertheless the sun continued to rise, the sky growing lighter and lighter blue. Dio's skin started to hiss and boil, bubbling toxically. Erina couldn't bear to look him straight on, the part of her that understood the origin of the body he wore felt tortured.
"You're going to die if you stay out here any longer," she told him.
Dio didn't seem to appreciate her pointing this out, but he knew it to be the truth. He slid onto the floor of the coffin. "Get out of my way," he spat at her. He was holding that bag tight at his side, like he was a child and it a stuffed animal he couldn't stand to part from. He lay on the purple cushions of the coffin he had made for himself, red eyes open and staring at the lightening sky.
Erina, while keeping the baby as steady as she could, climbed over the lid of the coffin and swung it shut on Dio, protecting him from the ever approaching sun. Dio watched the outside world disappear from his sight, and he fought down the unexpected claustrophobia that rose at the closed lid. Quickly, trembling with fear, Erina ran her hands across the seam of the coffin until she found the latch to lock it closed. She flipped it down, trapping Dio inside.
She knelt on top of the coffin, far less comfortable then she had been before, but infinitely safer. Finally she felt a soothing relaxation spread across her, and again she was able to cry for the man she loved and lost.
A few hours later a ship appeared, white sails on the horizon. Erina screamed for it until her voice was coarse and dying, but it was enough, and the ship came to her rescue. Erina and the baby were helped aboard the ship, warm blankets wrapped around them and questions asked rapid fire.
When asked about the coffin, she replied, "Within it lies my brother-in-law's corpse. He died a few years ago. We were planning to cremate him, but perhaps a burial at sea would be best." She forced a smile.
Within the coffin Dio froze. He pushed against the lid, but it wouldn't open.
Erina continued, "We could open it and take his body out, but I'm not sure he'd want to see the sun so long after his passing."
"…wryy."