For a few years, NARA had their "Archival Research Catalog" system online for researchers to use. This allowed some limited searching for information before a visit - not all finding aids were (or are) online, and even those that are may be incomplete. ARC was shut down and replaced with OPA - Online Public Access earlier this month, and the transition for me has been lackluster.
On the plus side, results are now persistent and you can link to or bookmark them. Previously,results were temporary based on a live database query that would close out after 30 minutes of activity, so you could never send results to a friend and say "hey, check this out!" I took to saving each interesting result as a Word Document for quick reference later.
This new one changes that, but it also really doesn't work well if you don't already know what you're looking for. If you didn't know that records are generally separated into titles such as "general correspondence," or "classified correspondence," when what would you search for?
I can, at least search for EVERYTHING in a record group at a specific location, but if I search for all of record group 19 at NARA II "textual records" I get over 15,000 results. I'm having some luck - I tried searching for "naval" in Record Group 181 at NARA San Bruno and then hit the "refine by series" link, which listed all 58 of the listed accessions they have information posted about, including one for the 14th Naval District (Hawaii, essentially):
Fourteenth Naval District. Office of the Commandant. General Correspondence, 1925 - 1957
So... learning by stumbling about.