At the other end of your flush hose is a check valve assy. #'s 14, 6, 8, 39, and 4
http://www.boats.net/parts/search/Honda/Outboard Engine/2005/BF225A5 XA /MOUNT CASE/parts.html
Most likely #8 is not seating correctly due to build up of salt, debris, corrosion, etc.
Sometimes, taking it apart and cleaning will fix it. Replacing the rubber o rings, gaskets, and possibly #8 will definitely fix it.
In the mean time....make sure there is a washer in the end that you would screw a garden hose to. It is just like a washer at the end of a garden hose. Part #3
http://www.boats.net/parts/search/Honda/Outboard Engine/2005/BF225A5 XA /EXTENSION CASE/parts.html
That should seal it up at that end.
Mike