How can we use your software to analyse a combination of tidal turbine and diesel power?

You could model your hydrokinetic turbine as a wind turbine.  You would have to create a new 'wind turbine' with the power curve of the hydrokinetic turbine you have in mind.  (It will show power output versus water speed.) Then you would need to enter hourly 'wind speed data' which would actually be water speed data.  I don't know where to get such data, but maybe you do.

Make sure you set the 'anemometer height' equal to the turbine hub height so that HOMER doesn't scale the wind speed data.