http://www.german-helmets.com/KREIGSMARINE%20Main.htm
I looked up this website when I was looking at painting helmets for Dragon's German Naval Infantry set. The kit box art shows blue helmets, but the website indicates the Kriegsmarine having standard green or grey helmets. So which is correct? Also, the kit didn't contain any decals, so are there any aftermarket decal sets with the correct labels?

There's just not a big enough demand for naval insignia to justify the research and production. You could try scanning commercial decals to a JPEG file and do a color change for the national insignia and print your own decals. On the other hand, the website you visited indicated that Kriegsmarine helmets didn't always have insignia. I believe after 1942 all helmets left the factory without any insignia at all.

Try this set: http://aber.net.pl/detal/desc/german_helmets__liners__chinstraps_1012590/
It has a few Kriegsmarine decals in gold.
Gary