Mamono Musume - Slime & Scylla

There is the world where humans and monsters live together. One day, a pirate, the main character, is washed off on the shore of small island... When he regains consciousness, there are two girls in front of him. "Are you a pirate? Human? Train us to be great marine girls!" "What...?" He is totally confused, but he later learns that the island has a legend about special treasure. Like this, their training life starts...