Kaizoku to Ningyoقرصان وحورية ; 海盗与人鱼 ; 海盜與人魚 ; 海賊と人魚 ; เงือกเจ้าเสน่ห์กับโจรสลัดหนุ่มสุดเท่ ; Pirate and Mermaid

From Aerandria Scans: During the time when the world is full of barren lands and big ships sail out to the vast oceans, a very rare type of sub-human--mermaid--was born. Normally, they have white hair, but only in water does the color change. Among the aristocrats, they are traded for large sums of money. Hisui is a merman with scarlet hair, and he's kept by a lord. However, as he's stolen away by the pirate Ikuta and gains freedom, they start their journey together!