There is so much going on here. The guy's grandpa has died, he moved to clean-up (and clean-out) her house, while living there with his aunt. There is also some "relative" girl (cousin?) there, but she is probably just a comic relief. The guy used to swim, but I guess not a swimmer, rather a "swim shorts club" member, whatever that means. Probably does not swim anymore for some reason. There is a girl, that looks very similar to Angel from Angel Beats, who is trying to learn to swim at night. There is a gren-haired girl that snipes everyone with a water-gun from like a mile away for them not wearing shirts. There is a violet-haired girl who would not mid doing it with the guy, if he is gentle at first, and who has a weird fox, that's either an alien or magic. Oh, and I guess the guy is recovering from some trauma, too, probably related to why he is no longer swimming.
What's more important there is this suspicious grandma, that's probably at the core of whatever is going on here, and there is definitely something going on, since the island looks suspiciously similar to the one from Summertime Render, so maybe this is a sequel to that. I don't know, it feels like they want to tell us a lot at once, and success depends on whether they will try to juggle all these things.
