It is possible to sail into Portsmouth Harbour but the entrance is very busy and you should not attempt this unless the wind and tide conditions are right to get in AND back out again. Note that the wind may be affected by the harbour walls and surrounding buildings.
You must use the small boat passage and there are a number of other requirements that you should be aware of – see www.royalnavy.mod.uk/qhm/portsmouth/regulations/small-boat-channel for details.
Due to some astronomical quirk, whenever the high water at Portsmouth is at midday (or thereabouts) it will be a Spring tide. Likewise, whenever low water Portsmouth is at midday it will be a Neap tide.