Not a Sea Cadet, but from what I've heard from an old JROTC buddy, the little bit of advantage you have goes away almost immediately. Everyone else picks up very quickly.
It is a little bit of a head start. At least you'll know how to salute, and you'll probably be giving your roommates some shoe shining tips. You might be a little more comfortable with the environment in general. You'll be used to some yelling. You'll be used to shutting up and keeping a face like stone. You'll be used to preparing a uniform and putting it on in three minutes. But it's all small stuff that everyone will be able to do at the end of Plebe Summer anyway. You'll just have an easier time with that stuff because it'll be a little less of a "culture shock".
From an admissions standpoint, I got a JROTC nom with a solid recommendation, and a bit of a push on the BGO interview (my SNSI apparently had a few chats with my BGO). I knew a bit more about the Navy than the average sociable high school student, and that ended up helping me during the Congressional interview; questions that were obviously supposed to be curveballs that the average student would've had to go on Wikipedia to look up I could answer because it happened to be in the NJROTC curriculum.
JROTC/USNSCC/CAP folks are the minority, and like I said the advantage, while certainly there, is small and vanishes as the average high school student gains the equivalent of your four years in JROTC in the six weeks of Plebe Summer.
