I wouldn't think there would be any need, your milk will adjust to provide more of the thirst-quenching fore milk - but you may need to feed more frequently. I BF my LO in Thailand where it was regularly that hot, and I never gave water. At that age she would have been on roughly a 3hr EAS, having her main feeds on WU, but on hot days I would offer the breast before naps as well or earlier if she was sweaty and fussy, and she might suck for a few mins or she might refuse. I figured those short feeds were her thirst feeds, and longer feeds were hunger.
If your LO is going through her usual number of wet and dirty nappies, you shouldn't have any concern with dehydration.
Don't forget to stay well hydrated yourself too. Enjoy France!