yes, it is variable...we used to go a lot in December.....
one year was magnificent and we swam every day...
the next year, i had to bundle the kids up in their snowsuits, hats and mittens.....it was cooooooooooooooold..........
as Miffy said, you have to pack for both possibilities....layering is probably your best bet.....light for warm, with layers added on as it gets colder...