He pretty much nailed it - in terms of ships, dentists will only be on the LSDs, LPDs, LHAs, LHDs, CVNs, ASs, LCCs, and of course the hospital ships - although that seems like a lot it's really only a fraction of the fleet. The crusiers, destroyers, frigates, submarines that make up the backbone of the fleet and the various replenishment/transport ships do not carry dentists.
Having a submarine surface unnecessarily for any reason is a pretty big deal - and not in a good way - dentists have gotten heat in the past for not treating submariners aggressively enough resulting in them having emergencies while underway. As a result, you'll see that DC officers will generally treat more aggressively ie: endo versus pulp cap, extraction versus conservative treatment, when faced with a decision of that sort in treating a submariner. There are some very experienced IDCs out there who may be able to handle such problems, but there are others who have minimal dental training and experience and thus the DC officer needs to take the utmost care to prevent any such situations from ever rearing its ugly head.
