Seirei-sama wa Haramigoro ~Kanojo o Sukuu Kozukuri Vacation~

Keigo was trying to get through his summer break without any romantic issues come up. However, during this summer, his uncle, involved in trading, returned home to Japan. He brings Keigo a charm: a small glass bottle, which contained a young, female water spirit, who he saves.

Separated from her home, the young spirit's very soul was on the verge of vanishing...and the only way to keep her soul from vanishing is to bear a child with a human being! Thus, she asks Keigo to impregnate her.

This is how Keigo came to live with a beautiful water spirit and have the very best (and very luckiest) summer of his life!

[From English DLSite]