Marine Blue
Summary
From M.N.M.:
Young Chaeyeon has just lost her father and suddenly ends up living on an island with her grandfather. That's where she'll meet a strange boy coming from the sea who will change her life. Apparently mute, the young man will quickly give proof of great intelligence. But his previous aquatic life seems to catch up with him...

















... and the second season of the Anime did a better job of pacing than I thought they would. I think they moved the Elf village arc from before hostilities with the Principality began to the inter-war period, aka the start of the second season, and used that to pad it out. Good call if that's what they did. In any event. Good season.