If you plan to be at Fratton Park this Saturday, then a good friendly place for pre or post-match is the Ship Anson pub on The Hard. It's very friendly, guaranteed to have a few Pompey supporters in there, and the lunchtime match on Sky Sports. It won't be crowded and service is quick...
http://gkpubs.co.uk/pubs-in-portsmouth/ship-anson-pub/
Trip Advisor reviews...
http://www.tripadvisor.co.uk/Restaurant_Review-g186298-d4242072-Reviews-Ship_Anson-Portsmouth_Hampshire_England.html
In the same area, and only a couple of minutes walk is Gunwharf Quays, with plenty of shops and places to eat and drink. But a pint will be twice the price of the Ship Anson. There's a posh marina inside Gunwharf Quays, where you watch all the comings and goings into and out of Pompey Harbour.
The Ship Anson is also right opposite to Pompey's Historic Naval Dockyard. It's a fascinating place, free to walk around, but obviously you have to queue and pay for tickets to get on attractions like HMS VICTORY, HMS WARRIOR, and the Mary Rose.
For getting to the match from The Hard, it's a simple case of jumping on any train. Portsmouth Harbour train station is right opposite The Ship Anson. Takes 6 minutes, and all trains stop at Fratton Park station, and then a 10 minute walk. Alternatively the No.1 bus from opposite the Ship Anson stops v.close to Fratton Park [takes 15 minutes]. A taxi from the Ship Anson will cost about £7.
Have a good trip.