Very pleasant, consistent summer weather normally between early May and the end of September.  Can be very hot at midday in August.  You certainly won't need coats and probably not even a jersey at night.

Spring and Autumn can provide swimming weather or showers - they're less predictable.

December to March can get cold (down close to zero at times, but snow is very rare on the coast) but also provides bright sunny days inbetween - some people swim all year round - but you need to be tough !