Kekkon shitai Tatsunomiya-san wa Jouriku shimashita

The manga centers on a good-looking college student named Nagisa Nii. She saves a stranded turtle by the sea, and she is brought to the Ryuuguu castle under the sea as the turtle's way of saying thank you. Nagisa meets Princess Otohime in the castle, and Otohime falls in love with her. Otohime follows Nagisa on land, and starts living next to her using the name Kameko Tatsunomiya. (Source: ANN)