It certainly won't hurt your chances. I attended summer seminar in 2007 and I had a great time. Your MIDN leader will write a recommendation about you at the end. It is a great way to see the Academy and what it is like being a MIDN. It also weeds people out. Lots of people get pushed into USNA by their guidance counselor or parents . They get their asses kicked. I didn't personally think it was too terribly strenuous, there was daily PT, and you take a PFT for your package. When we took the PFT they gave us a large turkey lunch before hand. I'm dead serious. I think it was a tactic. You also get to do a mini sea-trials. Come to think of it, it was a pretty fun experience

. One night, they take you on an 'underground tour.' Won't tell you anymore because it was actually my favorite part of the week. Think the most we ran was about 2 or 3 miles. There was a lot of drilling as well. Sadly, we didn't get to shoot any firearms. There is one night towards the end where they yell profusely at you for about an hour. It seriously rattled some people, but just remember it is just a game, and keep your 1000 yard stare, don't crack a smile, and you'll be alright.
You are certainly going to need to boost your ACT score. I had a 26 or 27, with a 3.9 gpa with heavy AP courses, and tons of EC's (eagle scout, black belt, varsity sports, debate team, etc) and I didn't get into USNA. This was in 2007. It has only gotten harder. If you are a minority, that can help. Also, get rec's from former/present military. It is unfortunate, but I think the biggest thing that matters with them is test scores. I have seen some great leaders get turned away, and instead they pick some idiot who lucked out on the ACT. Boost the ACT, take as many AP/IB courses as you can, extracurriculars , PT, and stay out of trouble. A B in AP Chem certainly is better than an A in Pottery or Drama.
Civil Air Patrol won't hurt. I did it briefly. Be sure to get leadership experience. It is all about leadership for USNA! CAP was a lot of drilling in a parking lot. Also, the Air Force has an entirely different culture than the Navy. If you like the USAF, consider the academy in CO Springs.
Also, I really wished I had applied to the USCGA. It is a great school, and you can fly. It is also not as hard to get in. Consider the USMMA academy as well. I visited the campus, and I really did not like it. However, you might like it.
If you are set on becoming a Naval Officer, APPLY FOR NROTC! I cannot emphasize this enough. Be sure you pick schools you are competitive at getting into. If you get a ROTC scholarship, and the school won't take you, you are SOL.
But yeah, just keep on trying to boost your stats and have backup plan(s). ROTC and USNA guys ultimately end up in the same place after 4 years.