I am seriously considering going there and am currently in the application process now.
If you're interested at all you DEFINITELY should try to attend the Summer Seminar. It's a one week taste of what life at the Academy will be like. Go here for more info:
http://www.usna.edu/Admissions/nass.htm
I went last Summer, and I had a blast! NASS was the one thing that really made me seriously consider going to the Naval Academy. The Midshipmen told me that after going through NASS, you'll know for sure whether you want to continue pursuing the Academy or another military option, or if you want to completely give it up.
Since you're a Junior, I'd suggest going to NASS this summer (applications for it start in the Spring) to see if you want to go any further. Yes, you can choose to be a Marine officer (if I remember correctly, it's a choice you have to make from the start; you'll be doing some different things during your summer cruises but otherwise your classes should be the same; then again, I wasn't really listening because I want to be a Navy officer!).
Remember that you must be well qualified physically as well as mentally. Get involved in sports and keep your grades up!
If you REALLY want to be a Marine officer, also consider a Marine option NROTC scholarship:
https://www.nrotc.navy.mil/
I know there's some real Navy & Marine officers around here, so ask some more questions and they'll try to answer them. Good luck!