Wizard,
From an Academy grad I'll tell you that if you REALLY want to be an aviator, go ROTC. I loved the Academy and I still do. We all F---ing hated that place, but we also loved it in the weirdest way, and since misery loves company, you'll make awesome friends, and find an appreciation for the simplest of freedoms. Those are just some of the good things about the Academy. However, getting a pilot billet is competitive out of there. I'm an NFO, but that's because of my eyes, pilot was never an option, so you're not hearing this from a bitter dude who didn't get pilot and wanted it. However, there are a couple bitter NFO's down here from the Academy who didn't get pilot. The ROTC guys pretty much get a pilot slot if they want it. OCS is tough to get into, especially if you just want pilot, there's a long wait and you'll have to do pretty well in college where ever you go. So basically, go to the Academy if you want to go to the Academy, go ROTC if you want to be a pilot.
Okay, that being said, make it your option. Apply to the Academy and get in first, if you don't want to go, don't. Your first question, all sports and extra-curriculars (ECA's) look good to the Academy. What's even better is being a leader in one of them. Being president of the basket-weaving club is about as good as riding the bench on the varsity lacrosse team. Being a varsity lacrosse captain, you're golden. So, any sport, any ECA, especially leadership postions within those, the Academy loves. Your second question about sea cadets: Important? No. Helpful? Yes. If you read the Academy's brochures, you'll easily find a statistic on what percentage of the incoming class was a sea cadet. If they're keeping stats on it, it must count for something. Your third question, put everything you can think of on your application. Remember in Van Wilder when the teacher handed out blue books for the final exam? Van said he'd need more blue books? Ask for more blue books. They work on a point system. You get points for everything you put on that application, so put it all down. Community service, sports, ECA's, things you led (retreats, service projects, etc. they love that stuff).
So then you get in. Now the option is yours, and you can then weigh out what you want to do as far as becoming a naval aviator. Good luck, bro.
Matt