RE: Can't get off the water with Float Plane
The tyipical modelers solution is to add more power. J-3 Cubs don't add more power. Try adding a strip of balsa about 1/4" wide at the step. Also try lifting one float at a time. CAP floats on a Super Cub fly differently than Edo Floats and lots of times we had to fly one float at a time off the water. With glassy water you may need to step taxi back and forth a couple of times to create some chop to fly from.
Also, when you abort the takeoff, apply full up elevator to slow the taxi. Sounds like you have plenty of power and experience. You just need a couple of old bush techniques.