Though more expensive, a combination of:
Penyffordd - Heswall Anytime Day Return (£3.10)
Merseytravel Saveaway All Areas (£5.30)
Total £16.80
may be more desirable if you're planning to use any public transport in Merseyside while there. Three bus trips (in total, not each) or one trip on the ferry, and you'll have made a saving.
The Saveaway will give you unlimited bus, train and ferry travel within Merseyside (and on Merseyrail outside Merseyside).
As you'll be purchasing the Saveaway from a TfW guard, you won't need to buy a £1 Walrus card for it, it'll be a normal paper ticket.