Your question doesn’t make sense. As THParent said, summer seminar has no impact on getting an appointment or not. As said, it’s main purpose is to give as many people as possible a taste of the academy in the hopes you will like it and apply there.
If navy is your #1 choice, then it sounds like you have already researched the navy. Going to their summer seminar won’t increase your odds of getting an appointment there, and it’s unlikely that it will change your opinion of wanting navy.
You however appear to be willing to apply to the Air Force academy as a backup in case you don’t receive an appointment to Annapolis. The LOGICAL decision then would be to attend the Air Force summer seminar and NOT the navy if both were offered. This way you’d have a better understanding of Air Force in case navy doesn’t offer you an appointment but Air Force does. This way you could make a better decision on whether to accept an Air Force appointment. Just because you’re offered an appointment, doesn’t mean you have to accept it.
It seems that you don’t need to go to navy summer seminar. It’s obviously what you want. What do you gain by going to the navy summer seminar? Are you expecting to be persuaded more than you already are? It seems like going to Air Force summer seminar is the logical choice, in case that is where you are offered an appointment. So you’ll know something about what you’re applying to.