Portsmouth (or Southsea) to Ryde Portsmouth fastest if you have a car - or gone by train as the trains go straight into Portsmouth harbour. Southsea fastest crossing as still uses hovercraft - but pedestrians only.
hover craft - southsea to ryde or the fast cat portsmouth harbour to ryde , if you have a car you have a choice of either portsmouth or southampton with pretty much the same crossing time.
Whichever route you take its the most expensive ferry crossing in the world apparently .. .. worked out on a cost per mile