My first semester (year maybe?) in highschool I had less than a 3.0 GPA with a C in chemistry, but I really got the fire in my gut to go to USNA and turned things around. By the time I graduated, I had a 3.98 GPA in high school with a 1270 SAT (old version). I was the captain of the football team (and also selected all region), captain of the baseball team, and captain of the track team (all state 800 meters). I was also in the Beta Club, and had a few pilot hours. I took all AP classes my senior year, and AP Anatomy and AP Calculus BC my Junior year. I was accepted direct to USAFA and not USNA. I chose to go to the Naval Academy Preperatory School (NAPS) instead. (Well actually after 1st BCT at USAFA, but thats a long story...
) So it can be done!
Hit the SAT prep books hard. One thing I did was take the practice SATs that were in the books. Challenge yourself in the most advanced classes you can, and work your butt off to do well. After all the Academy is not going to accept excuses if you do poorly in calc or chem plebe year, etc.
If you REALLY want to go to USNA and you're grades aren't quite on par, ask about NAPS or the foundation program. You basically do a year of prep school and when you're done if you have a 2.0 or higher you go to USNA.
My dad was the director of candidate guidance and sat on the admissions boards at USNA for 3 years, so if you have any more specific questions let me know and I will pass them on to him. Good luck.