Shiosai no Lily

Fifteen years ago, Alfred failed to protect his childhood friend from the hands of pirates. Consumed by regret, he dedicated his life to the sea and rose to the rank of a naval lieutenant. But one day when he boards a merchant vessel under siege by pirates, he finds himself crossing swords with the very childhood friend he swore to protect, Lilian. Thus begins a romance of misunderstanding between a straight-laced naval officer and a tomboy pirate!

(Source: Square Enix)