I've been living in Hossegor for 15 years. Water can still be in the 20°C/70°F range in September. Mornings (when offshore) can be a bit chilly. If you've got room in your bag, bring a long sleeve vest, a spring suit (short or long arms) and a 2/2 or 3/2 full. September is one of those months where you never know what the weather will be like, so best be prepared for anything! Water will often be a degree or two warmer if you head down to the Basque Country to surf the reefs and points...