I grew up in the Central Florida area (don't live there now) and learned to swim in lakes at parks that were designated for swimming. We also swam in creeks where we found little swimming hole areas that were fun. Mind you, this was 15+ years ago. We knew there were gators and/or snakes. I think the gator population was lower then, also. I remember when gators were protected. Now they have a hunting season for them.
Would I swim in a lake or creek now? Nope. If I have a pool to swim in, great. I never cared for the squishy feeling of the lake/creek bottoms. My brother once gashed the bottom of his foot open on a broken bottle hidden in a soft, muddy pond bottom. He had to be stitched up in layers and on crutches for months. Nope, not me.
I always saw the pretty sandy beaches at the WDW resorts as a nice place to take a romantic sunset stroll with your sweetie. I imagine when taking in the water pageant it's a good place to hang around or maybe get your toes wet. I wouldn't swim there, though. Too many pretty pools to partake of.
If you truly want that fresh-water feeling, central Florida has some nice springs where you can swim/tube. I couldn't tell you where the heck they are now but I remember them from when I was a kid. I'm sure info can be found. I distinctly remember Blue Springs. We always laughed at the name because the water was so cold we were sure that the name came from the color your lips were when you got out. The springs were always so dang cold. I don't think any reptiles could survive there.