I'll be honest, you're a little to young to worry this much about your vision. Finish highschool, do your best and worry about it when the time comes. PRK is something that probably should be done when the Navy says yes, you can get pilot if you have the surgery. If you worry, you stress and that doesn't help at all. Certain things are out of you control, at least for now, so worry about what you can control.
As for my eyes, I had my first flight physical in 92 and I could tell my eyes were going. I made the 20/20 cut but by my NAMI physical in Pensacola the next year, I was at 20/30 (probably stressed a bit as well). Luckily the cut was then 20/30 correctable to 20/20. From 93 to about 00, my eyes slowly became worse until they bottomed out at 20/80 (-2.00 as I recall). I then had PRK in 03 and my eyes have been perfect sense. I've got 20/12.5 vision in both eyes (actually slightly better with my right eye), better than I ever had growing up. I picked up my COD the other day at 12 miles with my mark 1A eyeball (confirmed by control and my TCAS). Best surgery I ever had and highly recommened to those who can do it.