Casual nudity at it's most casual extreme! - "Hey, let's go skinny dipping!" - "B-But... we don't have a pool senpai~" - "I know. But let's just pretend that we're on an Australian beach! It'll be a blast!"
I suspect that "BALL 38" may mean it's the 38th chapter of the manga - lots of manga have a titling convention like that, where each chapter is called "File N" or "Nth Day" or, to give more exotic examples, "Parts N" (Love and Collage) or "Nth Ball" (Oretama a.k.a. My Balls, which isn't the manga this page is from - I already checked.)
Also, I suspect from the dialogue of the girl on the far right that they actually are clothed in swimsuits, but for some reason he (and we) are seeing through the clothing. That would explain what she's referring to that she "made for use in Australia" and explain certain aspects of her pose.