Umi ni Ikitai Osananajimi - Oneshot

[MangaDex]

 

[Airzpool]

Why did I feel that it's not because he reason is that he want he friend go away but it's was just an excuse to play a game without disturbing.

 

[Coldlight]

If you check the Pixiv raws, apparently the next 3 artworks are basically bonus pages of this set after the last page. (NSFW)
The girl goes back to get her floatie, catches him in the act doing his business, lewd stuff happens.

So this is just another one of those Fanbox bait manga after all.