How much time would I need to prepare before enlisting to make sure I have the best chances of passing selection?
It really depends on what your current fitness level is, however, generally speaking I tell guys who are modestly in shape to give themselves at least 8 months to get ready.
How long should I train to tread water?
The old grad standard was 2 min 30 sec with wrists and ears out of the water.
How long have you been a PJ? Did you go straight in or did you have another specialty prior?
When I joined the Air Force, I went straight to Indoc and was a PJ for 12 years. No prior Air Force jobs.