I'll be in Provence in early October. Will it be warm enough to spend time at a beach.
On a beach - perhaps.
In the water - no.
Yes, you may stay on the beach, enjoy the amazing views but the water will be too cold to go swimming. We went to Provence in June 2016 with my sister and we fell in love with 2 beaches: Saintes-Maries-de-la-Mer beach (Saintes-Maries-de-la-Mer) and Plage de Pampelonne (Saint-Tropez). These are paradise places! Definitely worth visiting.