I just discovered the National Oceanic and Atmospheric Administration's Photo Library. http://www.photolib.noaa.gov/index.html

I haven't seen much of it yet, but my eye was drawn by some old photos of sailing vessels in their "Sailing for Science" catalog. They might make nice reference for decorative elements.

Here are the terms of use as given on the website:
Restrictions for Using NOAA Images

Most NOAA photos and slides are in the public domain and CANNOT be copyrighted.

Although at present, no fee is charged for using the photos credit MUST be given to the National Oceanic and Atmospheric Administration/Department of Commerce unless otherwise instructed to give credit to the photographer or other source.