If I voluntarily resign from the Air Force Academy, am I still eligible to attend the Naval Academy?
You don’t provide much context, which is your prerogative, including whether you are an actual USAFA cadet or doing some applicant what-if thinking while you are applying to both but not sure which path is for you, whether you can get a do-over.
If you are in your first two years at USAFA, then you know you can resign and walk away with no obligation. You are free to apply anywhere, as long as you meet the academic, medical, physical and age criteria. If you are struggling with any aspect of academics, conduct, military aptitude, honor, etc., that grass is not greener at USNA.
If you are a cadet in good standing and doing well, but realize what you really want is a career in the water, under the water, over the water, water-related and salty around the edges, you would have to learn how to articulate that in nomination interviews, BGO interviews, essays and be fully prepared to embrace starting completely over, because you know a career in the Navy or Marine Corps is your true calling. You might be able to validate some classes you took at USAFA or based on HS, but you would be a plebe and do 4 years. Your chances would likely be low, because the bailing from USAFA, for whatever reason, is a flag.
As noted by others, if you realize Navy or Marine Corps is your true calling, don’t limit yourself to USNA, explore NROTC, OCS, etc.
Finally, if you are a cadet and are being allowed to voluntarily resign in lieu of disciplinary action related to conduct or other issues which would probably result in involuntary separation, your military separation document, the DD-214, will have an “RE code” that determines whether you are eligible to serve in the military in any branch, ever, or could be waivered, or, free and clear to serve again, among a range of codes available. That will be a determining factor if you are a USAFA cadet.