Just so - park at Hythe Marina, return fare £3.20. Ferries run every 30 mins at 20 mins past the hour, 10 mins to. And they were selling Boat Show tickets at half rate on Monday.
Jim, we went on preview day via the park and ride, and at 09.30 drove straight in, straight onto a bus and arrived at the Show 10 mins later. The next day, Saturday, the queue for the P & R started at the junction with the M27!. rather than sit there for hours, drove straight to the multi story at the West Quay centre, which had no queue, and only half full (at around 10 am). Drove straight in and walked about 300 yards to the Show main entrance - cost for the day, £3.60. Previous years when we had the boat at Hythe, we did the same as you via ferry - its a good way to go. I don't think the P&R is a good idea on public days - they do not seem to be able to cope with large numbers of cars.
Very handy for gate "F" (nearest the pontoons), there is also the free bus service from the station to the ferry, just buy your tickets in advance (or a station).
As long as you weren't going on Tuesday (or was it Weds) .. the ferry was out of action in the morning. Don't know if they still do it, but a few years ago you could get a combined ticket on the ferry which enabled you to enter at the 'old' entrance by the pier, rather than having to walk all the way to the far North entrance.
If you are coming from the East and don't mind a 20 minute walk, another option is to park (free) at Warsash and walk over the Itchen Bridge and through the centre of town.