Dive in. I used to be a competitive swimmer in high school.
Basically, hit the water, swim like hell to get the blood pumping.
We did that even when the heater broke one Christmas break and we were splashing around in ~72F water on our 2xday practices. The coach was nice and let us go home half an hour early or so. 
…in body bags?
I consider anything below 60 or so cold. Growing up around Lake Michigan, I don’t believe the water gets much warmer than 70 F. 80 is positively warm to me.
We visited a spring in central Florida where the water is a constant 74F last year. For swimming, I waded in waist deep and then took the plunge. Later on, Marcie tipped the damn canoe over so I effectively dove right in. If there was a significant difference, or advantage, to either technique, I didn’t notice it.
LouisB, you know that the correct answer for Floridians is wait until May when the water temperature warms up. In the case of springs they should only be swum in when the outside temperature is cold enough to make them feel warm. Cold water should never be swum in.
Yes, but who goes outside when it is cold enough to make 74F seem warm?