Both of the programs you mention Sea Cadets and NJROTC are great youth programs. If the high school you attend has NJROTC, I recommend joining. But you can still participate with your local sea cadet unit.
NJROTC offers its cadets a significant advantage (over non-NJROTC applicants) obtaining a college NROTC scholarship, especially for minorities. 'Distinguished' NJROTC Units can nominate senior cadets to the academies as well (similar to the nomination authority your congressman and senators have). Go to www.cnet.navy.mil/njrotc/ for more info and to find a list of NJROTC schools in your area. My office phone is 714-567-4944 if you want to talk. I am a NJROTC Senior Naval Science Instructor at Santa Ana High School, Santa Ana, CA. I graduated from USNA in 1977, became a P-3 pilot, and retired a few years ago.