The ferryport is ideally situated if you are driving, it is at the end of the M275 and therefore keeps stressed up car drivers with family and caravan in tow away from the actual city. Unfortunately the powers that were didn't consider other access methods. For future reference the best train station to get to/from is Fratton.
From the ferryport ignore ALL cycle path signs, head towards Portsmouth and Southsea to get out of the port, at the entrance to the port is the Rudmore roundabout, take the third exit (ignoring the no entry off slip from the motorway), should be signed for Fratton and maybe Buckland but there is a chain hotel on it (I think Premier Inn but not sure), at the end of the road turn right, straight on for a mile or so, just as the road rises for the rail bridge turn left onto the access road for Fratton station.