Of course there are other options upon not receiving an appointment - one of those options would be enlisting.
I assume you mean really enlist -- not NAPS, where you're enlisted for the year you're there.
I would hesistate to choose this option unless your desire to serve your country right now is stronger than your desire to attend college right now. I wouldn't count on it as a way of getting into USNA -- it can happen but, equally if not more likely, it won't. Enlistment is a big time commitment -- maybe 4 years?
If you don't get an appointment (or offer to NAPS), I'd first suggest consulting your BGO. Sometimes, we have insight into what the issue(s) might be. That may influence your choice regarding other options.
One of my candidates faced this issue a couple of years ago. His grades, SATs, etc. were just not good enough for USNA or even NAPS/Foundation. He wanted to enlist in the Marines. His father (a friend of mine) wanted him to go to college. An impasse resulted until, voila, he happened upon VMI. VMI and the Citadel offer certain things in common with the SAs but have lower academic standards. Did NROTC at VMI and is working toward his dream of being a USMC officer. Everyone is happy.