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 
Timing of SWAN global output files (swan_HS.63, swan_TPS.63, swan_TMM10.63, and swan_DIR.63) derives from the timing specified for ADCIRC global meteorological output in the fort.15 run control file. Note that the SWAN calculation interval in the fort.26 must coincide with the ADCIRC meteorological output interval or SWAN output will not be written. The fort.26 allows for output of wave parameters or 1D/2D spectra at user-specified points. For SWAN+ADCIRC applications, point output specification will typically occur after the QUANTITY command and before the TEST command. First, the user defines a group of points (‘sname’ from the SWAN manual) by either identifying their x and y coordinates directly in the fort.26 or within a separate file (‘fname’ in SWAN parlance). Next, the SPECOUT command triggers spectral output, and/or TABLE triggers time series output of desired wave parameters and time steps. The following lines show an example from a fort.26 that requests both spectral output and wave parameter point time series: POINTS 'SpecPts' File 'SpecLongLat.loc' SPECout 'SpecPts' SPEC1D ABS '1DSpecOutABS' OUT 19640903.000000 1200 SEC SPECout 'SpecPts' SPEC2D ABS '2DSpecOutABS' OUT 19640903.000000 1200 SEC TABLE 'SpecPts' HEAD 'WavesOut.txt' HS TPS DIR OUT 19640903.000000 1200 SEC
In this example, longitude/latitude coordinates are specified inside a file called SpecLongLat.loc; the two SPECOUT lines request 1D and 2D spectra written to 1DSpecOutABS and 2DSpecOutABS; and a table of Hs, Tps, and Dir will be written to WavesOut.txt every 1200 seconds beginning 9/3/1964 at 00:00:00.