I won't get into a match over Marines vs. Army, and I'll agree that probably a majority of Docs aren't up to the challenge for a USMC tour. The Navy's biggest mistake is putting interns into USMC GMO billets that don't want to be there. The Marines will crush them, and next to a squared away Marine, they look like idiots. Unfortunately, there are just too many USMC billets to find enough people who want to go green. I think that's a shame. In a perfect world, I agree that former independent duty corpsmen (IDCs) trained as PA's make the perfect front line asset. There just aren't enough to go around. The SEALs are already moving to that system, and I think the days of the IDC are numbered. There is also a lot of talk about using PAs in GMO positions, but again, it's a numbers issue. Plus, a GMO, O-3 with 2 years of service is significantly cheaper than an O-3 PA with 15 years in.
As for DMO career paths, they are not as straight forward as they used to be. There are a few CAPTs that have been DMOs their entire careers, but this is not a viable option anymore. You will not make O-5 without a residency. There used to be a residency in Undersea Medicine, which was realy just an OCCMed residency with some hyperbarics, but this is essentially gone as well.
Generally, people do the 6 months of training after internship, then do one or two tours in the fleet as a DMO. Anymore than that is just too much time out from residency. Residency in whatever you want to do. Then, you do a payback tour as a specialist. After that is complete, you'll generally be on O-5, and in line for senior level DMO jobs. These are positions like the FOrce Medical Officer for Submarines/EOD/Specwar. Also, there are billets at the Experimental Dive Unit, the Dive School, Naval Undersea Medical Institute. The exchange billet the r90t talked about is actually an O-3 billet that has been filled by twilighting O-6's for years, I don't expect that to change. Generally, the SUBFOR/SUBPAC jobs make people O-6's. There is the head honcho job at BUMED for a DMO, that's an O-6 looking for a star.
You need to plan on jumping in and out of your specialty in order to make O-6. One tour as payback is not really enough. Usually, people alternate one tour as a DMO/one tour in their specialty.
You can also come to DMO or FS later. My roomate through NUMI was a board certified ER doc, who had done regular shipboard GMO tours prior to residency, then residency, dept. head tour at NH GUam, then came in as a DMO. SO you can come in after your intial payback if you're picked up for straight through training.