Here is a sample acknowledgment you may use when you publish work that uses Ferret. When you publish an acknowledgment of Ferret, we would appreciate that you contact us as well, so that we have a record of Ferret's contribution to your publication.
The author(s) wish to acknowledge use of the Ferret program for analysis and graphics in this paper. Ferret is a product of NOAA's Pacific Marine Environmental Laboratory. (Information is available at http://ferret.pmel.noaa.gov/Ferret/)