Ah, a poolside made for two. When there's nobody else around it may as well be a private pool, I suppose?
Also, he planned to teach her how to swim but ended up just having some Sweet Vanilla loving instead. Which is perfectly fine, there's always next time, after all...
Seems 'Sensei' still retains enough presence of mind NOT to 'cum inside' Miku-chan (probably at least until she graduates), even if she gives her verbal consent.