I don't know about for the reserves, but the gouge I got from the Sailors I encountered on cruise/PROTAMID was that going undesignated SN was pretty much the worst deal ever.
Because you haven't been to A school, you can't really do anything that's job specific and so spend most of your time doing working parties and miscellaneous, usually talentless and mindless tasks that need to get done (think chipping paint and cleaning things). So take that for whatever it's worth.
Let me also just add the caveat that going enlisted when you ultimately want to be an officer is not necessarily the best route, especially with the Navy. Commissioning routes through the enlisted ranks are extremely competitive crapshoots (STA-21, etc) and timing with training/deployments/whatever could potentially put you at or beyond the max age for the Academy if you intend to reapply.
If you don't get NROTC or Academy right out of HS but want to serve as an officer, then don't jump into enlisting. Cool your heels and try to college program at a school with NROTC and keep reapplying for officer programs. I understand the impulse to want to "get out there," but try to think beyond the next 18-24 months and to what your ultimate goal is.
Also, to the OP, you've posted an awful lot of threads about an awful lot of things asking some pretty basic questions. Google and wikipedia are your friends. Go be an information sponge.