Yes you can use a All Zone Saveaway on the train all the way from Chester to Birkenhead and also on the Mersey Ferry.
But, on the Mersey Ferry you need to break your journey - you can't go on the full River Cruise - but you can go from Woodside to Pier Head ferry terminal in Liverpool which takes 15-20 minutes and you get a really good view of the waterfront and the cathedrals etc on the horizon.
When you get to Hamilton Square station in Birkenhead turn left out of the entrance and Woodside ferry terminal is a short walk away. When you get to Woodside go straight to the ticket office, show your Saveaway and ask for a Boarding Pass.
You could stay on the ferry to Seacombe but you would have to alight there and wait an hour for the next ferry.
Or, after getting off at Pier Head and looking around Liverpool you could then take the ferry from Pier Head, via Seacombe without getting off, to Woodside where you can catch the train back to Chester.
Or just get the train back to Chester from Liverpool Central or Liverpool James Street stations.
And yes, you can buy your All Zones Saveaway ticket at Chester station. Make sure they don't sell you a normal day return from Chester to Liverpool because that also costs £4.60.
Edited: 13 January 2012, 18:44