Hickdive
Contributor
Semi dries are fine down to 10C, if you dive in a quarry then you need a drysuit as the temperature will go down to 4C. Anyone who dives in a quarry in anything but a drysuit in winter is a statistic waiting to happen..
The first dive I made in 2010 where the sea water temp was as high as 10C was on 21 June.
Wetsuits and semis are perhaps useful on the south coast but elsewhere they can be totally inappropriate.
In addition the actual dive is only half the battle. Get cold in a drysuit and once you're back on the surface you will re-warm fairly quickly. Get cold in a wetsuit or semi and you will stay cold or actually get colder unless it is on one of the rare warm summer days we experience.