Akimbo

One day, while walking on the beach, an old man and his dog Fujio came across a baby mermaid. He named it Aki-chan, but the old man had lost his wife early and had no children, so he knew nothing about raising a child. And what's more, it was a baby mermaid! He didn't know what to feed it or even how mermaids lived! So begins the old man's struggle to raise the mermaid on his own!!

(source: Kodansha, translated)