With my Dexy, he would swim anytime it was over 50 degrees. Sometimes he swam when it was colder, much colder and turned into an icicle dog when he got out of the water.
Selli likes a 60 degree cut off point.
Duffy once went swimming unintentionally when it was about 10 degrees, that was a bad idea, he prefers 70 degrees at least.
I think it is an individual thing. As long as they can run around afterwards, they are usually O.K. down to at least 50 degrees.