Visiting Atami in September

Hello Japan travelers 🙂
I'm going to Japan with my parents in September (5.-19.) and for the last few days the plan is to go to Atami and just relax (16.-18.).
Has anyone been to Atami in September and can tell me if you can chill at the beach (and is swimming in the ocean allowed?) and what we might expect from the weather?
The Atami Sun Beach website has "opening" times but that was for 2025. From what I could find the official swimming season is only untill end of august. Is that still the case?

Thanks for any tips 🙂

by Angel5378