For general information, see the ADCIRC site, ADCIRC FAQ, or the ADCIRC Wikipedia page. For model documentation not yet available on the wiki, see the ADCIRC site. New content is being continuously added to the ADCIRCWiki, and material from the main ADCIRC site will be gradually transitioned over to the wiki.
The fort.26 file contains the the SWAN runtime parameters as well as the "coupling" details between ADCIRC and SWAN. This file is required to run the ADCIRC+SWAN model. Note that the name "fort.26" is ADCIRC's name (and fortran unit number) for the SWAN input command file, typically named INPUT by default.
The format is detailed in the SWAN User's Manual, Chapter 3: http://swanmodel.sourceforge.net/download/zip/swanuse.pdf
See Casey Dietrich's great webpage on running ADCIRC+SWAN