I Saved Too Many Girls and Caused the Apocalypse: Volume 5 by Namekojirushi

After everything Rekka and the girls have been through and with summer fast approaching, Iris thinks its time for a beach vacation. And not just any beach vacation, mind you. An intergalactic one that will take them to a bright blue planet of endless oceans and perpetual summer. But the waters—and the secrets—run deep here on Berano. After a group of space pirates shows up to crash their vacation, Rekka will find himself wrapped up in a conspiracy involving the treasure of a mermaid princess.