Sea Fisheries has been tracking the sea temperature and this week it was measured at 18.2C – just 0.1C off the warmest ever and breaking the record for this time of year.
A Met Office spokesman said the warm summer, lots of sunshine and light winds had led to the record-breaking temperatures.
'When it is windy it makes the sea more turbulent, so the cold water is brought up from the bottom and mixes with the warm water at the top,' he said.
'So the light winds have been helping.'
While yesterday's strong north-westerly wind has been churning up the sea, the spokesman said there would not be an immediate difference.
'It takes a little while for it to change, but it will probably be on a downward slope now. We should have about another week.'
Swimmers have been making the most of the unseasonably warm water.
Despite yesterday's strong winds, shop keeper Matthew Cable, 45, was swimming at Cobo.
During the winter he swims at Beau Sejour, but since June he has been going into the sea about five times a week.
'I went in during the first week of June and it was definitely cold,' he said.
'My friend timed me and I was in about two minutes and 40 seconds. It is much warmer now, however it's never too warm in Guernsey.'