My grandfather was on the USS New Orleans (ca-32).
This ship, like so many deserving but ultimately forgotten parts of history, should have had it’s story played out on the silver screen (imo).
It had an absolutely miraculous journey from Pearl Harbor, onward. It was a “ghost ship” and nicknamed the “NO (such) Boat” because everyone thought the goddamn thing had sunk so many times.
I have read what is available online, along with two firsthand accounts in books (one was written in 1944 and had to be heavily censored) but I know there is information a layperson wouldn’t know how to find.
Where can I hire someone to learn all about my grandfather’s boat so they can tell me about it?
I think your first course of action may be to get assistance from a reference librarian. Most academic libraries employ librarians with a specialization in one field or another, and they're very adept in finding materials not available elsewhere. This is especially true for history, as many large libraries also contain archives, and there is often so much material there that only a fraction was digitized. Because you're talking about maritime history, it may be that you can even contact the Department of Navel Records, which are a part of the National Archives, directly.
Best part about it, these services are often free of charge.
There is at least one firm that is historians for hire (History Associates), but it tends to be for companies wanting to commission a formal history of themselves.
I suspect what you are really looking for is a research assistant, which is not the same thing as a historian a lot of the time (a research assistant would compile research for you, as opposed to trying to synthesize it into a narrative). The National Archives and Records Administration maintains a database of independent researchers for hire who are familiar with NARA records. If I were in your shoes I'd probably start there as an avenue. NARA is where official military records are kept and the natural starting point. Once you had those, you'd begin to have the resources to explore other avenues — naval records include, for example, crew lists, which could serve as avenues for further (non-NARA) research.
As a note, you'll never learn "all about it." There is an infinite amount of information to possibly know, and only a small subset of that is available. I don't really research naval history myself, but I worked on an exhibit at the USS Intrepid on a submarine from the Cold War, and that research involved spending about a week at NARA getting everything I could find on it (crew lists, communication records, official memos, photos of it from its development period), but that was only a tiny piece of the full "history" of that ship (the USS Growler). A fuller history of it, which required the labor of a lot of people other than me, included interviews with whatever former crew members were living and available (and their wives), as well as the broader context that allowed me to situate that ship in the larger context (a brief moment in the Cold War) that it operated (both politically, technically, strategically, etc.). And that is far from the "full history" — that was just enough for a museum exhibit, which is much lighter on content than a typical academic monograph (because for some reason people don't want to read entire books' worth of content while they are at a museum with their kids).
So you should have some specific sense of what you are really hoping for, knowing that there are going to be real limitations and a million possible hypothetical directions you could go in.