Current sea temperature in Cornwall
The map below shows the current sea temperature for the coastal waters of the British Isles in degrees Centigrade.
In general sea temperatures in the UK are colder in the north than in south as we would expect. This means that compared to the north of England you would expect the sea temperature in Cornwall to be positively balmy. Unfortunately it isn't quite so; in the summer the sea temperature is around one or two degrees warmer than it is up north whilst in the winter the difference can be a more significant 4 or 5°C warmer.
In general though, sea temperatures in Cornwall range from around 7°C during a cold winter up to 18°C in the warmer summers.
For a detailed breakdown on Britain's (and the rest of the World) sea tempertaures have a look at the Global Sea Temperature website