User blog comment:Diego Brando/Phantom Blood (Doubt about coffin)/@comment-1739368-20121204022753

I'm pretty sure she was in the coffin because she needed it to survive the explosion. The two best theories I've heard so far is that:

1. either after Erina was saved, the coffin sank to the bottom of the ocean where Dio crawled into it, since vampires or zombies can breath underwater.

2. Dio had two coffins prepared on that boat, since the one they pulled out of the water had his name on it,  while the one Erina was in didn't apparently.