There’s a good chance that the weather will be fine too. Maybe we have climate change to thank, but the summer warmth seems to last a little longer every year, and it’s not unusual to have beachgoing weather well into October.
It’s also worth pointing out that, while the sea may be warm, it might not necessarily be calm. Autumn tends to bring a livelier sea to Cornwall’s northern coast: not the best conditions for swimmers perhaps, but absolutely perfect for surfers.