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