If it helps as a datapoint, I was diving in the Lavezzi Islands off the coast of Corsica this past weekend (middle of June), so slightly north of Sardinia. Water temperature ranged from 23 deg C at the surface to 15 deg C at 32 meters. The thermoclines are very noticeable, I didn't get an exact read of the depth at which they exist but I was glad to be wearing a 7mm wetsuit with hood over neutrally buoyant thermal skins. Some of the other divers wore a 3mm beneath their 7mm wetsuit, and the dive club said some people go even further with adding another 3mm on top, for a total of 13mm thickness.