To The Sea by Mukoubi Aoi

A green haired girl comes to meet a dolphin who asks her to join it in the epic ongoing struggle of the Tritons and the Poseidons for the peace of the sea. Together, they set off on a dangerous journey across the sea.
An adventure story that was originally serialized between 2007~2012, To The Sea is the author's English language - and professional - debut

Note: Due to mature content, this title is recommended for mature/adult readers, and is not recommended for younger readers. This comic grapples with some of the existential issues of our time, and includes occasional scenes & depictions of events that may be disturbing for a general audience and younger readers (in areas related to violence, sex, mental health).  

By Mukoubi Aoi, Published by Glacier Bay Books