Ho hum yes; not many cheap marinas in Southampton Water. Best bet would probably be Kemps, up the River Itchen beyond Shamrock Quay (but most berths dry at LW), or Eastlands above Bursledon Bridge on the Hamble.
Further afield, Wicormarine at Fareham gets many favourable mentions on here, but it's not exactly in So'ton Water!
Facilities- would want water and electricity and sheltered berth preferable. Not an overnighter, a daytripper. wouldn't be a long term, would only need it in certain months since boat would go to med in the summer. Just asking because i was in ocean village yesterday, picked up the marina prices and it would cost £312 per month to keep a 6.5m rib there. Just wondering if thats good value for the south coast?.
if it is for a RIB and you do not mind drying out for a little while every day in the mud then the Itchen is a good bet, with a little research you may even be able to sub let a private mooring. For example I have a flat on the Itchen and have a mooring with it, of the 20 moorings only a handful are used by the owner/occupier some of the others are let informaly to others (not realy allowed but it happens).