Weather in Antalya is typical of coastal Mediterranean regions. Apart from in the mountains, the temperature never falls to below 10C and usually averages between 20 and 30C with little in the way of rain.
For the fairer skinned amongst you, be warned - this is a region with temperatures that can top 45C in the peak tourist months and deaths amongst Northern Europeans who have spent the day walking around in the heat are not uncommon. Sunburn can occur within 20 minutes at the peak of the early afternoon heat. However the climate is pleasant between October and April with a little rainfall during this period.
The sea temperature is suitable for swimming all year round thanks to the convection currents of the Mediterranean.