Navionics provides this FAQ at,
http://www.navionics.com/MarineChartFAQs.asp?FAQID=9

Where do you get the information you use in Navionics charts?

It varies by area, but most commonly from local hydrographic offices
and official government charts, such as NOAA, NIMA, British Admiralty,
USACE, DNR, NHS and USGS. We also use private sources such as Fishing
Hot Spots, LakeMaster and Imray paper maps.