Disclaimer: I am just a parent of a USMA Plebe so my info may be flawed. But since no-one like a BGO has replied, I'll try to give you some reassurance as best I can.
I do not believe admissions learns of it from DODMERB but they will find out about it when (if) you get an appointment and fill out the required DOD paperwork (exact form number slips my mind) - where you need to disclose it again (one of the many things you must submit before I-Day). I don't think it goes to a character review board during the admissions process however you may have to answer questions to admissions regarding your use after the DOD form is submitted. The key message there is to not attempt to hide it but admit you experimented and be prepared to answer any questions of the circumstances, frequency, etc. Truthfulness and accepting responsibility go a long way towards showing character.
How it affects an appointment will depend largely on how you answer the questions posed and the perception of the admissions board. Concealing it now could be the kiss of death to your career or graduation when it inevitably comes out later.
There is no DODMERB "waiver" for drug use; DODMERB does not make that determination. DODMERB deemed you medically qualified based on your medical history: in other words, there is nothing physically wrong in your background that would prevent you from serving the Navy or in the military.
