To supplement what other have already stated about PAO: This is quite possibly the most difficult designator for non-priors to get into, even more so than Intel. It has very specific, additional requirements that the other designators do not have, one of which is interviews with three Navy PAOs. They would be able to quickly identify and weed out applicants who are not fully invested in the designator. Furthermore, it's a small community that typically has less than a handful of openings each year, most of which go to MC's (if not all). Accordingly, it should not be seen as an easy alternative for someone whose age or academic/ASTB stats fail to meet reqs for other designators.
I'm saying this from personal experience as someone who had been preparing a PAO package for the 2018 board. And it was my first choice, not a fallback. An O-6 PAO kindly took me under his wing and provided valuable advice -- I think the main reason why he did this is because 1) I am very passionate about the PAO community, and 2) I have 15 years of solid marketing communications experience. This includes 10 years overseas, where I directed multinational, multilingual teams on global-scale initiatives for top-ranked Global 100 companies.
Another factor to consider for those who think PAO could be a fallback option: The PAO board typically convenes only once a year, and it is not unusual for it to be canceled. This is what happened to the 2018 board, which is why I said "had been preparing a package." While this was very disappointing, I still found the experience extremely valuable. In order to find interviewers for my package, I cold called/e-mailed a number of O-5 and higher PAOs who were complete strangers. (At the time, I had no connections in any branch of the military). The astonishing level of support and encouragement they gave me provided firsthand insight into their community and made me even more determined to join their ranks one way or another, even if it means a lateral transfer after serving several years in another designator. I actually learned about the board cancellation before my recruiter because one of the PAO's e-mailed me immediately after he saw the BUPERS notice.
Bottom line: PAO is definitely not a community for disinterested or passive candidates.