I am currently a UMO in a Spec Ops billet. I say "super fit" because dive school is tough. Much more so than any of your other options coming out of internship. Therefore, UMO tends to attract more of the physical "studs" than, say, flight medicine. Even the guys who end up with subs still have to go through dive school.
UMOs serve 3 primary communitites: subs, divers, and special ops. On subs, you are not actually deployed with the subs (except for short periods) that job goes to the IDC. Your main job is Sub Physicals and and answering by message any questions the sub IDCs have. With divers, you could end up at any dive locker anywhere. The ND is something of dying rate in the Navy as more and more of the work traditionally done by NDs goes to contractors. These billets, you will utilize more traditional "dive school" skills because of things like scheduled decompression, etc. With spec ops, you will be diving at 25' with 50' "excursion(s)". You may also be assigned to a Spec Ops unit whose primary job is not diving (e.g., MARSOC, SBTs, SEAL teams). Your primary job there will be to bottom line NSW physicals (because the MANMED 15-105 says that only a UMO can bottom line these). Your chances of playing with cool toys = high. Your chances of going downrange or actually using your medical skills in direct support of actual operations = zero. Again, that is what IDCs are for according to the Navy. That is the stuff they don't tell you about in the recruiting adds. There are also a smattering of research jobs at NSMRL and NEDU and training billets at NUMI and NDSTC, FWIW.